I won't just drop a link and run. I have my ideas too, well they do correspond to others views as well. I've run across myths/legends about "a time before the Moon" and I put much credence in them.
I believe it's a hollow metal sphere. It was placed here in a very strategic spot. As to why, I don't really know. I still suspect something to do with controlling tides, but that's probably too easy of an explanation. Could be Watchers or maybe a derelict Space Station from another civilization...

The Moon; what is it ?