Secret Places (4)
Open the enormous blast door near the beginning. On the right side of the door frame is a secret compartment containing atomic health.
The hatch that the Sentry Drones pool out of is actually a secret place. I have no idea how one would reach it without a jetpack. You could try hitching a ride on a flying enemy or something.
There's a ventilation shaft hidden in the same general area as the previous secret. The two gratings are almost indistinguishable in the image, one is on the far left, and one is near the center. On the right you can spot the door from the previous secret.
In the Battlelord room, the bottom right monitor can be opened like a door.
Face the nuke button at the end of the level. Now do a 180 degree turn and scan the wall on the right for a switch (as seen on the upper right corner of the image). When you shoot that switch, a hidden door will open on the left wall (check the lower left corner of the image). It's a tough jump but it's possible. The secret level nuke button inside takes you to Spin Cycle. You can see a bit of the alien hologram face in the left of the image.