I always assumed it was a Vulcan colony on Earth's moon, Luna.
Oh! Simple yet brilliant! I like that!
But why would the Vulcans do that - build one of *their* colonies in *our* star system? Where's the...logic?
I mean, the Moon hasn't been terraformed, not even in the 24th century. Remember that DS9 ep with the Red Squad cadets? One of them lived on the Moon, and she said that you still needed suits to breathe on its surface. So why would the Vulcans go to all the trouble of building a colony there? If it was one of their own moons, then I can see it, but it's ours. So what would they be doing there?