They're definitely hiding something very sensitive there. For a long time they denied the sheer existance of the base although the Russians made aerial photos in the 60's/70's. It was in the Clinton administration when they finally admitted the existance.
They're testing unusual flying objects there, this is for sure as well. There are many photos and films which document unusual lights in the sky, obviously test flights of these objects. Once you could even see them from Las Vegas. 
The only question is: what's the origin of these objects? Alien or terrestrial?
People like Bob Lazar make me believe the origin is indeed from outer space (tiny seats, unknown letters). But this could be disinformation. 
Maybe the technology goes back to the Nazis. They were developing plans for creating this kind of flying saucers as the ultimate weapon. There are construction plans for objects like "Vril" and "Haunebu". Google them, you'll find a lot of information although some people claim, this is just a conspiracy theory as well. 
For further information there are also many vids on youtube, like this one: