Inside the second room on the left, there is two windows. And some pretty stiff opposition. Inside #2 window, a few steps down, you can see a switch. When you press it, a door between the windows will open.

Clean the newly-opened room out, grab the yellow card and then go outside again and wait a few seconds. You _may_ hear a door shut. Then run, as fast as you can, inside again and turn left. As you step on the fourth step, a part of the wall has opened; that leads to the secret room you can see through window #1.

When you exit the hidden door to #1, another wall panel slides open ahead to the left. Run like crazy to reach it, before it shuts.Inside: more goodies, and some ill-tempered gits. Feel free to waste them. And don't go back out yet!

Instead of going the same way out from #2, get in the near-circular pond of nuclear waste (This IS Windscale standard!) - you will fit through the left-hand outlet, as seen when you face the door. Hurry, that waste destroy your health in no time! At the end of the waste canal, goodies. Stay there...

...And check the walls. One is a slightly different color; a secret door. Press the switch, pick up the goodies and ride the lift down to the room with the first switch.
It is quite important that you press the switch in this secret room; it raises the walkway to the secret level.

After entering the room with the blue door (Please notice the raised walkway from #4 in the waste pit while passing) go to the right around the toxic waste pit. A part of the wall is slightly recessed here, that's a secret door. Carry on a few steps - the threshold to the big room is the trigger for the door. You may be able to hear it opening over the gunfire and screaming imps.

Don't take the exit in the big room. You ain't finished yet.

Go back towards the entry room. At the waste pit, cross the raised walkway and enter the up-to-now hidden room; it will open as you tap the wall. After knocking off the initial opposition, a 'invisibillity' may help. Else, you will need to be fast on your feet...

After negotiating your entry with the locals, make your way to the very back, and the very last room. Before you hit the 'Exit' button, try the opposite, slightly discoloured wall. It leads to a goodie.

When you finally hit the 'Exit' button, you have found a secret level: E1M9 "Military Base".