This is a cave complex reached through a sewer grate south of the reflecting pool in The Mall. In many places there are radioactive streams of water leaking down. Originally home to a small number of humans who hid here seeking refuge, they apparently perished from the radiation or the Giant Ants and Mirelurks that chose to make it their nesting place.
A little ways after you start out, you will meet a fork in the road. Both tracks will converge later on, the right path eventually dropping into the left path from above. Following the path will lead to a circular chamber used by the Mirelurks as a nesting site protected by a high level Mirelurk (a Hunter, King, or both).