Cage full of marines:

On the detention center level, you'll notice 
captured marines all throughout the level. In
one room, there are a three marines in a cage
above a lava pit. Once you are on the top level
of this room, press the green button in the 
corner. The cage full of marines will slowly 
fall into the lava pit. Press the button again,
and the cage will come back up - now containing
armor!