The Pitt Underground is an area in the Fallout 3add-onThe Pitt and contains The Pitt's Power Plant. It can be accessed from the steelyard, near a wrecked train entering an inaccessible metro tunnel. The second exit leads to Haven's courtyard in Uptown.
Both entrances are initially locked or closed. The manhole in the steelyard can be opened with the Power Plant key whereas the doors in the floor between the Underground and Uptown can only be opened if you assist Wernher in Free Labor. If you do not assist him, you can find the key on Wernher's desk in his hideout. Once you get the key to the manhole in the steelyard, the door in Uptown close to the Haven will be unlocked, but in here you can only get to the first room, and then the way is blocked.
There are a bunch of trogs in the area, most appearing in groups of two or three. Close to the Steelyard entrance, two trogs can be seen cannibalizing some unfortunate person from behind some bars. Twice during the Underground's linear journey, when reaching a corner with enemies ahead of you, several trogs spawn behind you.
In the second large room, after you go down the ramp, you will see some red arrows on the ground. The arrows will lead you to a broken toilet surrounded by four traffic cones, but be careful, because under each cone there is a hidden frag mine. The mines are easy to miss entirely if you have the Light Step perk, as you will not cause them to go off, and they are hidden from view unless the cones are moved.
ps3 Sometimes the game freezes when you enter the manhole the first time from the Steelyard. If the same thing happens again and again, it can be fixed by entering from Uptown, and then going back to enter from the Steelyard. [verified]