Hidden within a ring of mountains deep within the high desert is Area 51, a secretive military installation part of the Nellis Military Operations Area. Area 51 is well known for its role in conspiracy theories, and has served as an military test facility since the 1950s.
Originally known as Site II or "The Ranch" it was where Lockheed created the first U2 spy planes, and went on to build the SR-71 Blackbird, the A-12 (or OXCART and likely the inspiration for many UFO sightings) and the F-117 Nighthawk. The base is in the strange situation of being perhaps the most famous secret military base in the world, and visitors have been known to attempt to get close to the base, and take pictures with telephoto lenses.
The area around the base however is patrolled by numerous pickup driving guards armed with M16s. Though allowed to use deadly force if someone does not halt on entering the base, there has as of yet never been such an incident. Mostly the guards call the local police who deal out 600 dollar finest to those curious enough to try and get close to the base.
Though for quite some time no reliable (untampered with) aerial images existed of the base, and it didn't show up on maps, with its fame and the recent explosion of private satellites, one can buy high resolution aerial images of the base from private Russian firms, as well as the firm IKONOS - also originally started under lockheed martin, but now a private company.
A new element of the bizarre, famous-yet-secret nature of the base, has cropped up as Nevada has begun to realize the tourist potential of area 51 and has named the adjacent highway The Extraterrestrial Highway."
While the base is said by conspiracy theorists to be the site of everything from time travel research, to an underground transcontinental railroad, and of course to secret aliens working with the government to reverse engineer alien technology.
While it is almost certainly true that much of the base is underground there is little evidence for any of the fanciful claims that people have made. Much more likely is that secret experimental aircraft of a terrestrial nature (such as the Aurora, or the OXCART before it) inspired UFO sightings, which is exactly what they are: unidentified flying objects.