In this corridor, the last dark notch on the right is the entrance to a secret. Beware of the locals, they aren't all that friendly.

Inside #1, another secret. You can hot-foot it across the lava, or take the other way around. Either way is unpleasant.

A few steps further down the corridor from #1, there is a wall opening on the left, leading to a toxic pool with a lava ceiling. I guess they hired a too cheap interior decorator. Jump in the waste and follow the wall to the right. Small ledge with a secret.

You can reach this secret in two ways: enter through the door seen in the picture, or drop in through the ceiling, when you explore the upper floors. Only drop in through the ceiling if you have stocked up on ammo. The secret itself is not very interesting, but please explore the rooms with lava floor behind it: a really big goodie can be found here.

This is in the room with the blue door. Walk into the 'U' shaped thing facing the entry, and the secret opens.

This is like #5, just on the other side of the entry. Open it by walking close to the secret door.

Hint: never stand right in front of this secret or the blue door when you open it. It's like that certain pub in my home town: you never know what or who comes flying out...