Groom Lake was used for bombing and artillery practice during World War II, but was then abandoned until April 1955, when it was selected by Lockheed's Skunk Works team as the ideal location to test the forthcoming U-2 spy plane.
http://en.wikipedia.org/wiki/Area_51#U-2_programThe territory around Area 51 began to be built in 1954, but recently in 1958 was when it received the biggest impulse. From this date up to 1968 it didn’t grow too much, because it was destined more to experiment with anti-gravitational airplanes. But from 1968 to 1998 it has grown five times. They have even changed the riverbanks in order to build.
In 1968 there were three underground levels, in 1988 there were seven and starting from 1998, fourteen.
There were places where the depth of the constructions reached more than two thousand meters.
These are enormous hangars where mother ships are hidden. There are even constructions underneath a lake. But don't confuse this type of constructions with those that serve as offices, laboratories, etc.
One can enter through a hidden cave protected by heavily armed soldiers. To get inside it is necessary to use armored elevators with titanium walls that go kilometers down.