The 隐没的下水道 is a quite small (and due to the many waste barrels, heavily irradiated) underground location, containing some feral ghouls and possibly a Glowing One or Feral Ghoul Reaver (with Broken Steel add-on). It can be reached from the northern part of Dupont Circle.
With patch 126.96.36.199 on the Xbox360 the game may crash upon exiting the Sunken Sewer.
Sometimes Followers can get stuck in the room with the barrels when trying to leave. They will not escape even if you fire/send them away. DO NOT go into the large room after the initial hallway opening with your companions. Have them wait in the hallway until you are ready to leave. If you already got them stuck in there, just tell to wait and push them all the way to the hallway.