After opening the very first door, finish your business and walk over to the left. There is a Medipak on the floor. When you move up to it, a door to a secret wil open right next to you.

...Then do the same to the right. Except, stand in the splatted-out body...

From the first open area, behind the columns in the middle, a small door. Inside this door, turn left and go in through the next door. There is a pile of bones and skulls, a secret door to the left of it.

At the winding staircase, about 1/4 in from the entry point, there is a secret door in the center of the staircase. Goodies inside. To activate the door, go to the top of the stairs, then back down again.

At the top of the winding staircase, a small skull door. Enter, go to the right. You will be in a toxic waste trench. Reaching a larger room, there is a misaligned texture; the door to the secret. It will open by itself as you approach.

Carry on across #5, and reach the concrete area with the near-circular toxic waste pool. Get in the waste. At one side a secret, at the other a teleporter out again.

There is a switch inside the skull door: it will raise a set of stairs. Take the stairs up. Walk through the sheet of flames, and you have the secret. This is a 'serial' secret, the next...

...Is reached when you step into the teleporter behind the flames. As soon as you step down from the teleporter receiver pad, it triggers the secret. But it doesen't stop here: the wall behind you...

... Is decorated with a lion's head: a secret door. Inside is a teleporter, leading to...

...The last secret in this level: the exit to the secret level - E2M9, Fortress Of Mystery.