No tombstone unturned
(Access all bonus levels in Zombies.)

Here are 6 locations.

1) Level 1 - by a house (top centre).
You must pick up either a bazooka or monster potion to break the hedge.



2) Level 9 - by toxic containers (top centre )
Again, use either a monster vial or shoot through.



3) Level 12 - save all cheerleaders and you'll be taken to the bonus level automatically.



4) Level 17 - behind cob-webs next to the room with a monster vial (top centre)



5) Level 22 - via hidden passage in a laboratory.
There's also a hidden, powerful flamer next to the bonus level pick-up!



6) Level 33 - in top left corner, in water area.