Gas Tap Achievement
Prevent gas from being released in Military Base.To prevent the gas from being released you must first go through about 3/4 of the 'Military Base' level. The room is real close to a checkpoint so if they do release the gas, you can die and start over.
You know you are in the room when, if you do not succeed in stopping them, a green gaseous substance starts flowing from vents in the ground. Once you are in the room, there is a ramp to the right and you go up it and then have to kill everybody that tries to make a run for the console to flip the switch to activate the gas.