A letter from the USAF...

The U.S. government does not explicitly acknowledge the existence of the Groom Lake facility, nor does it deny it. Unlike much of the Nellis range, the area surrounding the lake is permanently off-limits both to civilian and normal military air traffic. The area is protected by radar stations, and uninvited guests are met by helicopters and armed guards. Should they accidentally stray into the exclusionary "box" surrounding Groom's airspace, even military pilots training in the NAFR are reportedly grilled extensively by military intelligence agents. To read more click here: http://en.wikipedia.org/wiki/Area_51