National Security. If the public knew what was there, then other countries would as well.
Most likely a lot of prototype planes and drones being developed there. Most of the satelitte photos show some types of Drones, which are launched at high-altitude off of the back of a plane.. like the SR-71 Blackbird.
They could possibley be working with some type of hazardous chemichals (Might be the by product of some type of fuel, dunno), as there was a court case from Workers against the Gov't when people were exposed and they're hair / skin started coming off.
Its got a lotta heat over the years, so I dont think anything too amazing is being test flown there.. probably moved all the "UFO" looking crafts to Skunkworks or Hangar 18.