Black Rock Processing Control Bridge. If you keep following the signs you will come to a giant abyss. Turn around and take first left follow the pipe and jump down.
Open the door to the Black Pyramid room and pull the light cord three times. Go into the third room and pick up the key. Continue towards Black Rock Processing 2 Jump back over the non-spinning turbines and head downstairs to where you fought the Hiss.
Go to the premises leading to your goal and you will realize that the bridge has not been lowered.
If you continue to follow the signs you will come to a giant abyss. To get to the Black Rock processing area in Control you need to build yourself a bridge. First head for maintenance and then follow the signs to the Black Rock Processing area. Eliminate the enemies in the security room.