Near the beginning/middle of Surface Tension, after you go through the storm drain hatch. When you arrive at the mesmerizing vista, with jets flying above another hydroelectric complex in the distance.

What you need to do is to stay on the upper pathway until you get to the giant pipe you can slide down. I know it's hard to resist, but don't slide down.
RIP purple hat

You'll notice the tantalizing array of ammo and suit battery chargers in a room directly above the pipe. Go there.

There is a terminal on the side. Push the button. It sounds like nothing important happens, but you just opened the way to the shack

Backtrack to the beginning of the map, and climb down the rocks near where you first did, until you reach the lower pathway of the map. To your right should be a cave with a body, and some ammo in it. Go in.

If you followed all the instructions, when you enter the cave you'll be teleported to Dr. Horn's shack. When you are finished being there, you can take the exit teleporter to go back to the cave, but beware that you can't go back to the shack.

This is an easter egg made by Chris Horn (AKA Stormseeker), who is the lead level designer for the Crowbar Collective. He made this map in ST, and put it in as a little signature of his work. It is somewhat related to his ARG (Alternate Reality Game) that is, as of the time of writing, still ongoing.

Picture made by members of the ARG

If you are interested in cryptography or puzzles or participating in this, learn more here[forums.blackmesasource.com].