Trapdoor

The Trapdoor appears both in Fallout 3, and in Fallout: New Vegas. In Fallout 3 it can be found in Smith Casey's Garage, leading down into Vault 112. It can also be found in Fallout: New Vegas leading down into Mr. House's Sercuritron Vault. Both are activated by a small yellow switch on the wall next to the door. If the trapdoor locks after completing the quest the only way to reenter is by using the console command "TCL" or by using anti collision mods.

Bugs
PC In Sceintific Pursuit, sometime after exiting Tranquility Lane the Trapdoor will close, and will not open. This may be fixed by simpley restarting the game.

XBOX 360 PS3 The switch will sometimes say "PICK LOCK: VERY HARD" but will not open even if you succesfully do pick the lock.