Hidden within a ring of mountains deep within the high desert is Area 51, a secretive military installation that exists as part of the Nellis Military Operations Area. Well known for its role in conspiracy theories, it has served as a military test facility since the 1950's.
Originally known as "Site II" or "The Ranch", this mysterious base is where Lockheed created the first U2 spy planes. They went on to build the SR-71 Blackbird, the A-12 (or OXCART, likely the inspiration for many UFO sightings) and the F-117 Nighthawk at this location, shrouded in secrecy and deep in the desert. The base is perhaps the most famous secret military base in the world, and for decades determined visitors have been attempting to get close, yearning for a chance to unlock its secrets with their telephoto lenses.
Consistently thwarted, the snooping citizens are unable to get close—the area around the base is patrolled by numerous pickup driving guards armed with M16s, allowed to use deadly force if someone does not halt on entering the base. As of yet, there has never been such an incident, but it's surely a deterrent. Usually the guards call the local police, who dole out a $600 fine to those curious enough to try and get too close. The closest legal vantage point of the base is at the top of Tikaboo Peak, around 26 miles away.
For quite some time no reliable (untampered with) aerial images existed of the base, and it didn't show up on any maps. With its fame and the the existence of private satellites, one can now buy high resolution aerial images of the base from private Russian firms, as well as the firm IKONOS—a private company originally started under Lockheed-Martin.
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, naming the adjacent highway The Extraterrestrial Highway", with nearby towns awash with extraterrestrial key chains, T-shirts, and an alien-themed motel called Little A'Le'Inn. See what they did there?
While the base is said by conspiracy theorists to be the site of everything from time travel research, an underground transcontinental railroad, and of course a secret lab where aliens work with the government to reverse-engineer alien technology, there is little evidence for any of the fanciful claims that people have made. It is almost certainly true that much of the base is underground, but its more likely use is just as exciting—a secret experimental aircraft testing space of a terrestrial nature—many of which could easily be considered unidentified flying objects.