Of course, part of me suspects that the Air Force built this giant triangle/bullseye just to excite the UFO crowd.
Eh.
Bombing range target.
<shrugs>
The eye in the pyramid, of course.
What sort of crops are those, do you suppose? 
What Bos said. The Groom Lake facility is within the Nellis Weapons Range, and, like, pilots bomb things there…
The bullseye actually is a calibrated target used to test guided bombs and missiles.