I believe it was a cave, and I know it was filled with ghouls, and the reason I didn't explore it right away was that the radiation was very high, almost instantly crippling and worse than any other area I've seen in the game besides vault 87. I know it was on the overland map, not in the downtown DC section, and it was not accessed through any sewer or anything, but I don't know if it was a named location or not. If I'm not mistaken I found it on the western outskirts of the DC area, west of the Potomac. The atmosphere of the "cave" is that the walls appear to be made of rock, and the floor I remember having a catwalk appearance, but I think the whole area was layed out flat, without any stairs or different levels. Finally, the radiation was very light when you first enter the door from outside, but it gets exponentially worse as soon as your begin moving deeper into the area.
If anyone reads this and knows what I'm talking about, or even has a guess, I'd appreciate any help. The radiation was so rediculous that I put off exploring until I had a better radiation suit and a stock of RadX, but I eventually forgot where it was and I have never been able to find it again.