Go left from the start, inside the door and left again, past the big stone column. There is a few goodies behind that green, thick wall. When you get the goodies, a door will open in each end of the wall. More goodies, and some unpleasantries inside.

A few steps further on you will stand in front of a blood pool - either right in front of the blue key, or you can see the blue key at a distance. Get to the distant pool. There is a skull switch; when activated, some walls in the stone column will drop. More goodies and unpleasantries.

Get behind the blue doors, jump down to the underground maze. Go right. Second corridor from the left leads to a 'dry' area with a big goodie; this is the secret. The teleporter behind it is just a way out again.

After getting the red key, go through the gate to the blue doors; take the blue door to the right. Inside the red tower in the corner, a red door with a teleporter leading to the last secret area.

There are no secrets in E3M8: Dis.