Used to be the airbase where they kept the captured/stolen/legitimately purchased russian aircraft. It was basically an airforce base operated by the CIA. The squadron trained in russian tactics and flying russian jects where there, the SR-71 and its predecessors operated from there, and its where a lot of secret aircraft testing has happened.
There's no aliens there, there's no alien spacecraft there, we've got russian spy photos of it, you can go onto google maps and see it if you want. Its just an airbase where they play with new toys and other peoples toys.
They're just so damn secretive about it because our military is so dependant upon technological superiority.