It's A Vent, Silly

On the North-West and South-West corner of the Capitol Building grounds in DC, there are a couple of little round structures. I asked around a few times, and it turns out that they are air vents for all of the underground stuff near The Capitol. Now I know. This one is the NW corner. I thought I took a picture of the other one, which is much darker, but I can't seem to find it right this second.