Rock Creek Caverns

The door opens onto the hot and dusty landscape of June in the Capital Wasteland. Specifically, onto the interior of what was once a halfway-decent barn. These days a good half the slats that used to form the walls are gone, and the roof's mostly collapsed- but, hey, it has a door.

(Also it has the carcass of a giant scorpion, but that's neither here nor there.)

"All right, guys," Ellen says. "This seemed like the safest place to start, out of everyone's way."