In the room with the angled stairs, look under the stairs for some goodies.

In the same room, just a floor lower - down in the pipes - another secret hides in the water underneath the ledge.

In this room, you have to start the big drill with the two switches to the right. Shoot the baddies, then go to the hole opposite the entry corridor. Shoot the back wall in the hole. This opens a new exit to some goodies, and to the 'Drilling Area'.