Thanks, guys....it was about 5 feet tall and at least a 12 foot diameter. I figured it had something to do with water. It was at Kripalu, which is a giant facility that can house probably 1,000 guests...it wasn't near the buildings themselves but out in a very green field. And yes, Gene, it housed lots of bird nests and spider webs....which is why i didn't inspect further.
EDIT: I just looked at some seepage tank images and they only have one or two holes....maybe this one had a lot because the underground mechanism is so big? Could just one tank handle a facility that big? I didn't see any others around.
At first I thought maybe some kind of composter?
Seriously, I know NOTHING about this kind of stuff....I didn't even see a real horse until I was 22! lol