At the very start of the level. In the left hand wall, hidden behind two barrels is a small recession.
Reward: Pistol

Still at the start of the level, the far wall has a cracked section that can be destroyed by throwing a nearby gas can into it. If the can is already destroyed, there's an alternative method to access it, read on.
Rewards: a big shortcut, Shotgun, 24 shotgun shells, 24 pistol bullets, large medkit, small medkit, bag of gems

Back on the "intended" path. In the second room after the level start, the one with wooden boxes, in the far corner opposite the entrance there's an odd wall that can be walked through.
Reward: Super Shotgun, 24 shells, small medkit

The same room, after passing through the shed in the center, turn around and check the dark corner of the far wall. The vent grate can be destroyed.
Reward: diamond

Right after encountering the cultists for the first time. The three wooden doors can be destroyed.
Rewards: 12 pistol bullets, small medkit, bag of gems, two beds

In the room with the "Something below is calling us" wall writing, under the stairs there's a switch that opens a hidden path.
 
In the secret area from #2 there's an odd wall that can be interacted with, right opposite the wooden beam with the switch and ladder.
Reward: Fast Fire Totem
 
On the second floor of the house, near the toilet.
 

At the start of the level, the closest hay bale (with rifle ammo leading up to it) can be interacted with.
Reward: Assault Rifle

At the base of the tower near the barn there's a cracked wall segment with a helpful gas canister right beside it.
Rewards: 24 shotgun shells, bag of gems

In the shower on the second floor of the house by the pond.
 
Use a barrel (or any other method) to get on top of the corn field maze. In the far right corner, there's a secret stash.
Rewards: 2 rivets, 10 gold coins

In the same area from #3 there's a rectangular recession in the ground that can be interacted with.
Rewards: Riveter, 3 rivets, 2 large medkits, 4 potions, a basketball

This one doesn't count towards the total. In the secret #4 area, to the left of the teleporter, there's a basketball tucked in the corner.
 
Pick it up, carry it to the house by a small pond (opposite the barn), and put it into the basketball hoop on the wall to open up a secret passage.
Rewards: Mortar, Claw (Dusk HD only), Crossbow, Assault Rifle, Sword, 2 Fast Fire Totems, Hallowed Health, 5 rivets, 30 crossbow bolts, 120 assault rifle bullets

The teleporter from "Rachael's secret area" will put you right on top of it.
 
Alternatively, you can place a barrel on a nearby tractor's roof and carefully jump onto the roof of the barn from there. 
Reward: Hallowed Health
 
In the garage with the red key, in the far right corner, hidden behind a barrel and a stack of tires is a switch.
 
It opens the door in the secret #4 area (the trapdoor on top of the corn field), which leads to the first hidden level.
 
● There's a fast fire totem on the roof of the garage at the end of the big fight. You can rocket jump, crossbow boost, prop hop or just stack several barrels to get there. 

 
● You can jump over the level exit gates to find a secret message and a hallowed health. 

(Alternatively, from the tunnel that leads to the exit, you can slide into the ladder and hold the jump button. That way your momentum will be preserved and you will be catapulted from the top of the ladder and over the gate.)
 

The pond near the hut with a pistol, with a scarecrow hiding behind it. On the bottom there's an underwater passageway.
Rewards: Yellow Key, Assault Rifle

The first yellow door you encounter, to the right of the ladder that goes down to the sewers.
Requires the key from secret #1.
Rewards: 3 Rivets

Inside secret #2, there's an odd piece of wall that can be interacted with.
Rewards: Hallowed Health, Soap

Another yellow door in the same location as secret #2, opposite the ladder to the sewer.
 
Alternatively can be accessed by diving in the drain pipe room, navigating the underwater passage and then rocket jumping / crossbow boosting up the vertical shaft.
Rewards: 4 Rivets, 72 Assault Rifle Bullets, 36 Shotgun Shells, 4 Bags of Gems, Large Medkit

Right after entering the blue hatch, there's a switch hidden in a recessed space at the base of the ladder.
 
It opens a trapdoor in the next room.
Rewards: 12 Shotgun Shells, 24 Assault Rifle Bullets, shortcut to the secret #4 area

● Just like in the previous level, jump over the exit fence to find a hidden message. 

The graveyard to the right of the level start. Behind one of the tombstones there's a switch which opens a door in the crypt where the yellow key is.
Rewards: Super Shotgun, 12 Shells, 2 Small Medkits, Bag of Gems, 3 Coins

The tunnel under the church. There's a gas can lying right in the middle of the path, and a destructible cracked wall a short distance further down.
Rewards: 12 Shells, 2 Rivets

Underground, the room with the assault rifle. It's in one of the recessions on the wall.
 
Underground, the corridor from which a swarm of rats comes out. In the far left end, hidden behind two wooden barrels, there's a cracked wall.
Rewards: Blue key, Diamond, 3 Rivets

The room with the rectangular well, near the red key. There's a portion of the wall with a small opening and a coin in front of it. It can be interacted with.
Rewards: 72 Assault Rifle Bullets, a rat
 
Underground, after passing through the blue door, in the room with "the scourge of olden times" wall writing. The cave-in can be walked through.
Rewards: Diamond

● The graves at the cemetery can be dug up.

Rewards: 2 Coins, 2 Shells, 1 Rivet, 2 Small Medkits, Bag of Gems, 3 viscera

● The church windows can be broken, allowing you to get inside without the yellow key.


● It's possible to completely bypass the church and half the underground area. At the red gate, behind and to the right of the church, jump onto the red post and crouch-jump on top of the gate itself to get to the other side.


● In the garage with the blue door, there's a diamond hidden in the corner behind a bulldozer.


● In the Secret #3 area, you can jump off the ledge to your death to catch a glimpse of a secret message on the wall.


● In the boss room, the son of Intoxigator can be seen in the water behind the bars.

 
● To my knowledge, this is also the only level that has a point of no return in it - I couldn't find a "legitimate" way to get back out after falling into the boss area.
 

At the very start of the level, on top of a barrel.
 
The room with the super shotgun and yellow key, where you get ambushed by cultists. On the piece of machinery in the center of the room, hidden behind two barrels, there's a switch. It opens up a path back to the start of the level, as well as a secret area.
Rewards: 12 Shells, 24 Assault Rifle Bullets, 2 Bags of Gems
 
Right after going through the portal. There's an odd wall directly under the wall writing, which can be interacted with.
Rewards: 48 Shells, 48 Assault Rifle Bullets, Gold Coin

In the secret #2 area. The wall at the end of the corridor can be opened.
Rewards: 3 Small Medkits, 3 Bags of Gems, Gold Coin

In the secret #3 area, yet another wall can be opened.
Rewards: Riveter, 10 Rivets, 48 Assault Rifle Bullets, 24 Shells, Large Medkit, a dopefish cameo

● You can shoot the switch in the portal room through the grating, opening it up from the outside.

 

From the beginning of the level, go left and check inside the train car.
 
Further down the path is a segmented building missing a wall. Climb onto a nearby hill and from there jump onto the roof.
Reward: Hallowed Health

The same building. Underneath the leftmost section is a switch that opens a secret path.
Rewards: 72 Assault Rifle Bullets, 2 Large Medpacks, 4 Grenades, Crossbow, 10 Bolts, 5 Gold Coins, shortcut

In the secret #2 area, across from the teleporter is an interactable wall.
Rewards: Mortar, Bag of Gems

Right next to the red key. The cistern can be climbed into from the back.
Rewards: 4 Rivets

You can climb onto the big hill in the center of the map, starting from where the rusted railroad bridge is.
Rewards: Fast Fire Totem

● You can hop across the edge of the pit or use the jump pad at the bottom to reach the far side.

Rewards: 2 Rivets, 2 Gold coins
 

Right at the start of the level, there's a cracked wall that you can blow up.
 
If you don't have explosive weapons, there's a gas can a little further down the path, on the rail tracks.
Rewards: 20 Crossbow Bolts, 48 Assault Rifle Bullets

In the secret #1 area, dive into the water and swim to the right.
Rewards: Hallowed Health, Soap

At the end of the rail tracks there's a grate with the yellow key behind it. There's a small side path nearby to the right. Follow it and dive into the water.
Rewards: 2 Bags of Gems, Diamond
 

One of the cylindrical silos has a hole in its rear side. Get inside and drop into the underground tunnel.
Rewards: 2 Bags of Gems, Super Shotgun, 2 Small Medkits, 20 Gold Coins, literally every other secret in the level

Use the tunnel from the previous secret to reach the inside of another silo. There's a switch on the wall which opens up a hangar near the blue locked gate.
Rewards: 5 Rivets

In the same underground tunnels, there's a cracked wall near the T-intersection. It can be blown up with a gas can which is inside one of the silos accessible from the tunnel.
Rewards: Pistol, 4 Bags of Gems, 24 Shotgun Shells, 36 Pistol Bullets, 8 Gold Coins

Within the secret #3 area, there are golden coins in the shape of an arrow that point at an interactable wall. Behind the wall is a small maze which opens up into a large area with some enemies and a teleporter.
Rewards: Riveter, 8 Rivets, 48 Assault Rifle Bullets, 5 Hunting Rifle Bullets, Small Medkit, 3 Gold Coins

On the second floor of the house, in the bathroom.
 

At the very start of the level, behind the super shotgun is a cracked wall that can be destroyed with a nearby explosive barrel.
Rewards: shortcut, 1 rivet, 5 hunting rifle bullets

Inside the secret #1 area, the wall behind the teleporter can be interacted with. Another teleporter there will take you past half the level. If you're following the guide, don't drop down there yet.
Rewards: a bigger shortcut, Assault Rifle, 5 rivets, 96 assault rifle bullets, large medkit

Enter the teleporter from secret #1.
Rewards: Shotgun, 2 large medkits

In the open area with a building in the center. On the ground level, across from the yellow door, hidden behind some rocks is a small cave entrance.
Rewards: Sword

At the entrance to the building with the red gate, there's a transformer. Look behind it.
Rewards: 5 hunting rifle bullets

Across from the red gate there's an interactable wall. Inside there's a teleporter that lets you bypass the red gate.
Rewards: shortcut, 10 crossbow bolts, 5 hunting rifle bullets

Right after passing the red gate, on a shelf.
 
Enter the teleporter from secret #2 and break the vent grate to go outside.
 
Alternative method: after opening the gate to a forested area, jump on the wooden box and crouch-jump onto the fence.
 
Follow the fence to the right and break the vent grate in the wall.
Rewards: 5 hunting rifle bullets, 24 assault rifle bullets, 24 shotgun shells, 2 large medkits, bag of gems, hash

On the rear side of the building with the blue gate is a door.
Rewards: 3 rivets, 5 hunting rifle bullets, small medkit, 2 bags of gems

● In the secret #3 area. Shoot the switch (don't press it as it will open up the floor under you and drop you into the secret #4 area). Climb the newly revealed ladder.


Enter the duct maze and find a switch in there.


Now return to the ladder and drop all the way down.

Rewards: Riveter, 3 rivets

● Before entering the forest area. The switch that opens the gate also opens a wall inside the building you came out of. Come back inside the building and go downstairs.

Rewards: Yellow Key, Hunting Rifle, 2 large medkits, 2 bags of gems, a teleporter back to the start of the level
The yellow key is an alternative method to access secret #3.

● On the roof of the building from secret #7 there's a secret message, reachable by wall rocket jumping or crossbow boosting.
 

● Rocket jump or crossbow boost over the fence at the end of the level to find another hidden message.
 

Inside the "Books and beer" shop.
 
There is a switch on the second floor balcony.
 
Pressing it unlocks a trapdoor on the ground floor.
Rewards: 4 coins, diamond, a teleporter
The teleporter will allow you to telefrag a Fork Maiden if she hasn't been alerted.

On the roof where the teleporter from the last secret put you. Alternatively, you can access this roof by a ladder. Break the grating behind the water tank and drop inside.
Rewards: Pistol, diamond, 2 large medkits, 24 handgun bullets, 1 rivet, a teleporter
The teleporter will allow you to telefrag a Scarecrow in another building.

In the building with the Yellow Key, on the ground floor, interact with an odd segment of the wall.
Rewards: Super Shotgun, 36 shotgun shells, 3 rivets, 2 beers

The wall across from the Yellow Key building has a cracked segment. Use a nearby barrel to blow it up.
Rewards: Shotgun, 2 bags of gems, 2 large medkits, 24 shotgun shells, 24 pistol bullets

Inside the previous secret area, there is a cracked wall in the corner near the shotgun.
Rewards: Soap, 2 small medkits, 24 shotgun shells

The gas station windows can be broken.
Rewards: Assault Rifle, bag of gems, 7 gold coins, lots of beer and hash

Across the street from the gas station there's a garage.
 
Check behind the cars.
Rewards: 10 crossbow bolts, 24 assault rifle bullets, small medkit

To the left of the "Books and Beer" shop, there's a bricked up garage that has an opening in its side wall. Use a nearby crate to climb into the opening.
Rewards: 5 hunting rifle bullets, 48 assault rifle bullets, 3 grenades

In the sewers after passing through the yellow door. In the purple area with the pipes, across from the shotgun, is a switch.
 
It opens up a small alcove in the wall further down the level.
Rewards: Red Key

To the right of the "Books and Beer" shop, there's a back alley with stairs leading to a door.
 
Use the key from the previous secret to unlock the red gate inside.
Rewards: Crossbow, Riveter, 20 crossbow bolts, 5 rivets

● From the Secret #2 roof, jump onto the brick wall and shimmy towards the corner of the garage, at which point you can squeeze yourself out of bounds. Wandering around there, you will eventually find a secret message and a teleporter back.


● In the room where the big fight takes place, you can reach the ledge with the rivets by strafe-sliding into the jump pad.


● The longer you spend in the level, the faster the clouds in the sky will move.
 

Next to the hunting rifle, there's a small rectangular recession in the wall. Interact with it.
 
In the tunnel that leads to the exit, interact with a section of the right hand wall.
Rewards: Failed Experiment
 
● You can reach the quarantine release button through the fence without opening the door.

 

From the start of the level, turn left. Behind the barrel, the wall of a structure can be interacted with.
Rewards: Riveter, 4 gold coins

After entering the open area, turn left. Use a nearby barrel to jump over the fence and into the electrical equipment area.
Rewards: Pistol, 2 rivets, 10 crossbow bolts, 12 shotgun shells, large medkit

As you approach the installation, instead of entering the gate, turn left and follow along the left hand wall.
 
At the end of the path, interact with the odd segment of the wall. The teleporter inside will take you to a small forest area.
Rewards: Crossbow, Hallowed Health, 2 cans of hash, shortcut to the center of the grainery

Inside the grainery, under one of the two cars in the garage. Blow up the car to reveal it.
 
After falling into the silo with the yellow key. You can dig up a pile of grain to reveal a grate.
 
Break it and jump inside.
Rewards: 10 crossbow bolts

In the area from the previous secret, use the climbing thing to scale the back wall and keep navigating the passageway.
Rewards:
72 pistol bullets, 12 shotgun shells, 4 small medkits, another climbing thing, fast fire totem

Under the roof of the awning before the yellow door. Use the climbing thing or make a trick jump from the roof to reach it.
Rewards: 2 rivets, 6 gold coins

Use the climbing thing to reach the top of a mast.
Rewards: Hallowed Health, a secret message

● In the secret #3 area, you can crossbow boost over the wall to get out of bounds. Wandering around there, you can find a secret message, a sword and a bunch of blue keys. To get back on track, go back the way you came, climb a nearby rock, jump and crossbow boost back in.

 

Right after starting the level, while going down the stairs you'll find a cracked wall. You can break it with a gas can from the next room over.
Rewards: 5 hunting rifle bullets

In the back of the previous secret area, there's another cracked wall.
 
There's another gas can hidden in a corner further down the level, to blow up the wall with.
Rewards: Hallowed Health

In the Artefact Storage area, there are breakable vent covers. Climb inside, follow the path and break the grating at the end.
Rewards: 5 hunting rifle bullets, 12 shotgun shells

In the dark tunnel. As you approach the red key and get spotted by a wendigo, a section of the tunnel wall will open up behind you, leading to the area behind the bars.
Rewards: Mortar, 3 rivets, large medkit

In the final room, on top of the furthest storage rack on the left. Blow it up to reveal the soap.
 

After defeating the very first two soldiers, turn around, break the vent grate and climb inside.
Rewards: Pistol, 48 assault rifle bullets, gold coin

Inside the crawlspace from secret #1, interact with the wall segment where the gold coin was, right before the turn.
Rewards: Super Shotgun

In the area with two turrets, use a nearby box to jump up to where the turrets were and break the vent grate.
Rewards: 5 hunting rifle bullets, 24 pistol bullets

To the left of the yellow key, there's a cracked wall segment.
Rewards: 1 grenade, 5 hunting rifle bullets, 24 assault rifle bullets, large medkit

In the same area, right across from the previous secret, there's an interactable wall.
Rewards: Claw (Dusk HD only), 2 grenades, 10 crossbow bolts, 72 assault rifle bullets, Hallowed Health, large medkit

Inside the previous secret area, to the right of the entry point there's a switch which opens up a hidden alcove nearby.
Rewards: Diamond, madness crystal, soap, Dopefish cameo

At the very end of the level, inside the thresher, just before falling down a waterfall of blood there's a switch at the far wall.
 
It opens up a secret area behind you, at the ladder's base.
Rewards: 4 bags of gems, 2 large medkits, 2 grenades, 12 pistol bullets
 

The area where a wall opens up to reveal an ambush. On the lower level of this area, in the near right hand corner, is a breakable vent cover.
Rewards: large medkit, bag of gems

The same area, again on the lower level, in the center of the back wall there's a semi-hidden crawlspace with crushers in it.
Rewards: Hallowed Health

At the top of the "don't shoot the buttons" shaft, there's a breakable vent right under the ceiling. Use the climbing thing to access it.
Rewards: Hunting Rifle, 5 rivets, a diamond

After passing through the blue gate, in the room before the yellow key. There's a switch behind the barrels in the corner which opens up the gate in the floor.
 
There's soap on the top of one of the containers.
Rewards: 36 pistol bullets, 12 shotgun shells, 5 hunting rifle bullets, 10 crossbow bolts, 4 rivets, large medkit, soap

Again in the shaft with the climbing thing, as you go down the stairs from the top, there's an unlockable door near the lower level.
 
Enter the door and descend the shaft. At the bottom there's a cracked segment in the floor.
Rewards: 9 grenades, small medkit

In front of the exit gate, there's a cracked segment in the right hand wall.
 
The path takes you to a large arena filled with all manner of pickups. There's a button on the far wall, press it and use the teleporter to return.
 
Now navigate back to the secret #5 area. The grating in one of the broken pipes is now open, revealing the secret level entrance.
Rewards: a bit of everything, secret level
 
● Between two yellow gates, you can hop onto the fence window and reach the hatch release button through the fence, skipping most of the level.

 

In the hall with the cylindrical reservoirs, one of them isn't filled with lava.
Rewards: large medkit, 48 assault rifle bullets

In the large lava chasm, right beneath the entrance there's an opening over the lava's surface. You can try to access it immediately or wait until you get the Thermal Suit later on in the level.
Rewards: 2x Hallowed Health, a diamond

Within the previous secret area, in the far left corner there's a semi-hidden crawlspace. It's difficult but not impossible to navigate it without a thermal suit.
 
There's soap tucked in the corner.
Rewards: Sword, thermal suit, diamond, 3 small medkits, soap

In the large, vertical area with the climbing thing. After picking up the climbing thing, scale the wall to the right or the left until you reach the pipes under the ceiling.
Rewards: 24 shotgun shells, 5 hunting rifle bullets, 10 crossbow bolts
 

Shortly after the start of the level, in the room with a wendigo. Interact with the wall in the corner.
Rewards: Pistol, Assault Rifle, Fast Fire Totem, 6 rivets, 24 assault rifle bullets, 12 shotgun shells, large medkit

On a table in the room with two pistols.
 

In the area behind the superhot serum hall. Shoot the button through the fence.
 
This opens a door on the lower level that leads to the area behind the fence.
Rewards: Shotgun, Hallowed Health, 12 shotgun shells, 3 rifle bullets, bag of gems, 4 gold coins

Immediately after coming out of the spiraling pathway, there's an interactable wall segment on the right.
Rewards: Sword

In the same area, at the end of the crawlspace there's an interactable wall.
Rewards: Riveter, 48 assault rifle bullets
 

Opposite from the entrance into the reactor area, there's a building with the yellow key inside. Use a nearby jump pad to jump onto the roof.
 
Go inside and check behind the section of a wall to find a ladder.
Rewards: Crossbow, 5 hunting rifle bullets, 20 crossbow bolts

By the same building, in the lava fissure on the ground, there's a teleporter that leads to a secret area.
Rewards: Riveter, Fast Fire Totem, 6 gold coins

In the building in the far right corner of the reactor area.
 
A wall by the entrance can be interacted with.
Rewards: diamond, 4 gold coins

In every corner of the reactor area, there's a jumppad. Each of them takes you all the way up to a catwalk that encircles the entire area.
 
On the catwalk, from the far right corner of the entrance, jump onto one of the girders that bridge the reactor towers and follow it uphill.
 
Once you reach the tower, use the nearby crate to reach the top.
 
The thermal suit will allow you to explore the lava fissures. Make note of where the secret #2 teleporter is located so you can reach it before the heat protection ends.
Rewards: Thermal Suit

From the previous secret, drop down onto the girder that leads towards the center of the area, then drop down onto another girder that leads up to the far right reactor tower.
Rewards: diamond, 20 crossbow bolts, 12 grenades

Inside the secret #2 area, start bunnyhopping and enter the portal at a slight angle to the right.
 
In the air, keep strafing forward-right and with good enough speed and angle you'll land onto a nearby reactor tower.
 
From there, navigate the girder towards the powerups visible in the distance.
 
Alternative method:
From the same place where you dropped down from one girder to another to reach secret #5. Crossbow boost yourself onto the nearest reactor tower, then onto the girder which leads to the top of another tower.
Rewards: Hallowed Health, 8 rivets, Soap

Here's an overview of the level with the entrance and secrets #4, 5 and 6 marked down:
 
In the building with the blue gate. There's an interactable wall segment near the entrance.
Rewards: Serum of Blistering Heat

In the open area, in the building with the switch that unlocks the exit elevator, there is some loot hidden behind the crates in the corner.
Rewards: 3 bags of gems
 

In the main building, go down the right hand stairs.
 
Flip the switch on the back side of a pillar.
 
It opens up an alcove in a smokestack behind the main building.
Rewards: Climbing Thing

Use the climbing thing from secret #1 to climb on top of the building with the yellow gate.
Rewards: Hallowed Health, soap

On the same roof from secret #2, towards the back.
Rewards: 12 grenades, 20 crossbow bolts, 24 shotgun shells, 3 small medkits

Inside the yellow gate building, use the climbing thing to climb to the first alcove on the right.
Rewards: 10 crossbow bolts, 4 rivets

Same area, climb to the third alcove on the left.
Rewards: Crossbow, 2 small medkits

Behind the yellow gate building, there's a madness crystal.

 

From the start of the level, enter the opening and make a U-turn left.
 
Go to the far end of the dark passageway.
Rewards: Shotgun, 2 bags of gems

On a ledge above secret #1.
 
In the winding stairway area, where the hunting rifle is, make a jump from the stairs and into a red walkway.
 
Follow it and climb the ladder.
Rewards: large medkit, diamond

In the same stairway area, a little further down, there's a breakable vent on the wall.
Rewards: 13 shotgun shells, 5 hunting rifle bullets, 6 grenades, 2 large medkits, 2 bags of gems

After breaking your flashlight and picking up a new one, turn back the way you came.
 
Go through the door in the corridor.
 
Behind the boxes in the corner, there's a hidden switch.
 
It opens up a secret pathway in the same room.
Rewards: Red Key, 10 crossbow bolts, large medkit

After reaching the bottom of the spiraling staircase with a wendigo, keep turning in the same direction and check under the stairs.
Rewards: 12 shotgun shells, 1 rivet, small medkit, bag of gems

To the left of the exit door, there's a teleporter leading back to the start of the level.
 
In the secret #2 area, you can now unlock the red door with the key you got from secret #5.
Rewards: Hallowed Health, 8 rivets, diamond
 

On the rocky outcrop with the red key, blow up the barrel and jump into the revealed teleporter.
Rewards: Blue Key, 20 crossbow bolts

In the area with grated floor and cultists, after dropping down one level, unlock the blue door with the key from secret #1.
Rewards: Assault Rifle, Fast Fire Totem, 3 grenades, 48 assault rifle bullets, 12 shotgun shells, diamond, 2 large medkits

In the area with the "they drove us mad" wall writing. At the exact spot where the teleporter places you, there's an interactable wall.
Rewards: Hallowed Health, 16 grenades, 2 bags of gems

After picking up the yellow key, on the ledge with the three cultists there's an interactable wall segment.
Rewards: 20 crossbow bolts

In the purple circular area, break the vent grate.
Rewards: Mortar, 6 grenades, diamond, large medkit

In the back of the boss arena, there's an interactable wall marked with three coins.
Rewards: 5 hunting rifle bullets, large medkit, soap, a teleporter back

● As you reach the teleporter to the boss arena, walk around it and enter it from the other side for the Mother! achievement.

 

As you enter the main hall, look right and crouch-jump onto the pipe.
Rewards: 5 hunting rifle bullets, 2 bags of gems

In the same area, there's an interactable piece of the wall over the pipe.
 

At the start of the level, directly behind you is a cracked wall with a coin in front of it.
 
To the left of the cathedral entrance there's a gas can to blow it up with.
Rewards: Crossbow, 10 crossbow bolts, diamond

To the right of the cathedral entrance, behind the corner.
Rewards: 2 bags of gems

Shortly after entering the cathedral, under the spiraling stairs.
 

In the red key room after the ambush. One of the walls to the left of the entrance is interactable.
Rewards: Mortar, 12 grenades, large medkit

After passing through the red gate, behind the altar.
Rewards: diamond

After passing through a corridor with fireballs. Both pathways from the room have interactable walls that open into the same secret area.
Rewards: Mortar, diamond

In the crawlspace that can be accessed either from the pit with the madness crystals or from the blue door corridor.
 
There's an ammo pickup that counts as a secret.
Rewards: 10 crossbow bolts

After passing through the blue gate, strafejump from the horns over the altar to either of the openings under the ceiling.
Rewards: Super Shotgun
 

After navigating the lava and reaching solid ground, there's a cracked wall behind an explosive barrel in the area with cultists.
Rewards: Mortar, 12 grenades, large medkit

In the open area with lava falls. There's a medkit behind the very first one.
Rewards: large medkit

As you navigate the lava falls area, you'll pass through a small cave. At the bottom of that cave is a cracked wall behind an explosive barrel.
Rewards: 6 grenades, diamond

After finding the climbing thing, climb the left wall to reach a ledge near the top.
Rewards: diamond

In the corridor with many cultists, at the far end there's a crawlspace hidden behind wooden barrels.
Rewards: Pistol, 36 pistol bullets, large medkit, bag of gems, crystal of madness

Behind the first blue gate, after coming up the stairs, it's on a broken wooden beam in the wall.
 
In the same area, you can crawl under the wooden beam and towards the left into a hidden area.
Rewards: 2 bags of gems

In the corridor with the second blue gate. To the left there's a broken section of the floor with a ledge on the other side. You can make a precise jump or crossbow boost yourself there.
 
Alternatively, you can drop down onto that ledge from above later in the level.
Rewards: 10 crossbow bolts, 2 large medkits

Behind the red gate, down in the lava there's a hidden alcove.
Rewards: Hallowed Health, bag of gems

● Inside secret area #1, there's another cracked wall in the very back, above eye level. It can be blown up with a nearby barrel for a shortcut to the upper level.
 
 
● One level above secret #5, in the corner with a dead soldier, hidden behind wooden boxes there's a crystal of madness.
 
 

The pillar in the center of the town is hollow.
Rewards: 4 bags of gems

Towards the back, enter an alley behind a wooden barrel.
 
Go behind the house and drop into a waterway.
Rewards: 12 shotgun shells, large medkit, 5 gold coins

From the bridge with the red key, turn left and use the jump pad to reach the top floor of the building to the left.
 
There's a crawlspace leading inside.
Rewards: Super Shotgun, diamond, 2 small medkits

From the jump pad near the crossbow, land on the roof of the building from secret #3.
 
Next to the topmost red gate, there's a switch. Pressing it displays the message "a house opens".
 
The house in question is near the center of the town.
Rewards: Serum of Blistering Heat, Climbing Thing, 24 shotgun shells, 48 assault rifle bullets, 2 large medkits, 2 madness crystals

Grab the climbing thing from the previous secret. Go to the end of a long tunnel opposite the house, enter the hole in the wall and climb up.
Rewards: 4 rivets, 3 large medkits, 2 bags of gems

Use the climbing thing to scale the wall with stained glass windows at the back of the village.
Rewards: Hallowed Health, 5 hunting rifle bullets, large medkit

From the previous secret, climb up to the top of the chapel.
Rewards: Riveter
 

In the area with multiple cells and the sword. Open the first cell on the left.
 
Interact with the back wall to open a secret alcove.
Rewards: 10 crossbow bolts, 5 hunting rifle bullets, 2 small medkits

Same area, open the first cell on the right.
 
Check behind the wooden pole.
Rewards: 12 shotgun shells

In the first open area with cultists. Jump into the meat-covered opening behind the assault rifle.
Rewards: 3 bags of gems

Same area. There's a button behind the pillar next to the assault rifle.
 
It opens up an area on the ground level.
Rewards: 20 crossbow bolts, 2 large medkits, 2 bags of gems, madness crystal

Walk through the yellow gate and immediately turn left.
 
Behind a broken stone slab, there's a switch hidden in the dark.
 
It opens a pathway directly ahead.
 
Soap can be found in a corner to the left.
Rewards: Claw (Dusk HD only), 3 rivets, diamond, soap

In the secret #5 area, walk down the stairs and through the red gate.
 
Stand on top of the toilet and flush it to be transported to a secret level.
 

Jump into the fake lava pool.
Rewards: Blue Key, 3 rivets

In the red key room, crawl into an opening on the left.
Rewards: 3 rivets, 2 small medkits

Again in the red key room, press the button at the base of the entrance.
 
This opens a pathway just outside.
Rewards: 2 Shotguns, Hallowed Health, 24 shotgun shells, 6 grenades

Behind the yellow key, there's a button hidden in the corner between bookshelves.
 
It opens up a section of the wall in the same room.
 
There's soap on a ledge over the opening.
Rewards: 6 grenades, diamond, soap

In the boss arena. Let the boss hit you and launch you up to a circular ledge above the arena. You need to crouch in midair to fit in there.
If the boss is already defeated, you'll have to crossbow boost, mortar jump or prop hop.
Rewards: 2 large medkits, 3 bags of gems
 

On a wooden barrel near the edge, directly behind the cathedral.
 
After descending the spiral staircase, in the room with the diamond and the fast fire totem. Crawl underneath the stairs.
Rewards: large medkit

In the room with a lava fall. Check behind the wooden barrel.
Rewards: 2 bags of gems

In the same room, in a dark corner under the stairs, there's a hidden switch.
 
It opens up a wall on top of the stairs.
Rewards: Hallowed Health

In the secret #3 area, check behind the wooden barrels to the side of the entrance.
Rewards: diamond

● You can use soap to break away parts of the cathedral and get inside immediately. You can also throw soap at the altar in the back to break it open, without having to summon the tornado.

 

Directly down and to the left of the starting point, it's in the garbage container.
 
On a platform behind and to the right of the climbing thing.
Rewards: Riveter

Use the climbing thing to scale the wall to the right.
Rewards: Serum of Blistering Heat, large medkit

On the lower level, there's a breakabale vent cover.
Rewards: bag of gems, 8 gold coins

Inside the vent from secret #3, break another cover and climb out of the vent. There's a switch hidden by the base of the ladder to the left.
 
It opens up a grating across from the ladder.
Rewards: 20 crossbow bolts, 3 rivets, diamond

In the right wall of the railway tunnel, there's a crack covered by a couple wooden boxes. The boxes - or even the wall itself - may already be destroyed in the shootout by the time you get there.
Rewards: Hallowed Health
 

Behind the mass of rocks visible from the starting point. Use the jump pad nearest to the start to land on top of the rocks.
Rewards: large medkit, diamond

On the roof of the barn. Aim for the roof as you use the jump pads to reach the island the barn is on.
Rewards: 30 crossbow bolts, 2 bags of gems

In the bathroom of the house where the message "How can I be here?" appears. You'll find it in different places around the room or even the house, depending on how much the gravity has been switched around before.
 
After you switch gravity to be upside down of how it originally was. Fall down and aim for the white glow to land on a small island.
Rewards: 40 crossbow bolts, 4 bags of gems, a diamond
 

To the left of the cathedral entrance, there's a gas can. Bring it with you.
 
After falling through several floors, you'll see a diamond. Behind it, there's a cracked wall.
Rewards: 24 assault rifle bullets, 2 large medkits, bag of gems

In the room with the soldiers, on the right hand side behind the metal struts there's a breakable vent cover.
Rewards: 24 shotgun shells, 48 assault rifle bullets, 2 bags of gems

In the house area, go outside and make a U-turn to the right. Use the jump pad to reach the roof.
Rewards: 2 large medkits

In the giant forest area. From the entrance, turn right and follow along the wall. Once you reach the corner, interact with the section of the wall behind the shotgun shells.
Rewards: Littleneck, a backpack full of ammo

In the next area after the giant forest. The wall in the far left corner is interactable.
Rewards: Hallowed Health, 4 rivets

In the same area, after coming down the stairs, the soap is inside the tiny house surrounded by tiny trees.
 
This one doesn't count towards the total. Use the soap to break the brick wall in secret area #5.
Rewards: 2 Fast Fire Totems, Hallowed Health, backpack full of ammo, 2 diamonds
 

From the start of the level, turn around and you'll see a teleporter far off in the distance. Drop down and walk towards it in a straight line.
Rewards: crystal of madness, level skip

In the starting area, walk forward and you'll find it at the base of one of the trees.
 

In the house with the yellow crystal, in a cupboard right next to it.
 
During the final fight, on the ground between two platforms.
Rewards: Hallowed Health, 24 shotgun shells


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