DCTA Tunnel 014-B Potomac

is part of the Metro system under the Capital Wasteland.

Notable loot
In the southernmost chamber (behind an underwater passage) is a D.C. Journal of Internal Medicine, guarded by a Mirelurk King.

Glitches
Noticed in the Xbox 360 version of the game, the water near the Mirelurk Eggs of the Georgetown West exit sometimes glows a highly fluorescent toxic green color. Confirmation needed from other users

Confirmed with more details. It was normal at first pass (entered via Arlington Utility), but upon exiting to Georgetown and immediately returning the almost white glow was throughout the area. So it may be re-entering from bright daylight that causes the glitch.