https://uschs.wordpress.com/2012/01/26/secrets-of-the-capitol-grounds-olmsteds-summerhouse/

This place is near the Capitol Building. It's amazing. It's probably related to the underground tunnels.

But the reason "they say it was built" was because when the National Mall was constructed and people started touring it. There was a complaint of no where to get water because merchants were not allowed in that area of the Capitol Building grounds. So they made this place.

Use it to refill dromedary bags/camel packs and water bottles if you find yourself walking around that area.