There have been numerous reports of unusual sightings in the area. Weird lights flashing in the sky. Unidentified flying objects hovering in the air. Hikers and pilots mysteriously disappearing. The strange phenomena and super-tight security have given rise to many conspiracy theories. Some are listed below; take your pick.
Some people say that years ago aliens crash-landed at Rosewell, New Mexico, and were taken to Area 51 to be studied. Some of those aliens may still be alive and living underground at the installation.
It's also possible that the US military holds meetings with extraterrestrials at Area 51 and may be involved in joint human/alien plans to take over the world—and beyond. As part of this effort, the US military is developing top secret devices for weather control, time travel, and teleportation, as well as cloning alien viruses to which humans have no resistance (mad cow disease, anyone?). They say there's an entrance to a world-wide underground railway system at Area 51. There's also talk of the Cheshire Airstrip, which appears when workers spray water on it and disappears (like the Cheshire Cat's smile in Alice in Wonderland) when the water dries. This procedure, developed with alien technology, is perfect for hiding a landing site.
One prominent conspiracy theorist, Bob "Hee Haw" Carter of Reno, Nevada, suggests that the US government wants people to think Area 51 is swarming with aliens so we'll be too distracted to notice its real purpose as a secret research center for a powerful international organization that has nothing to do with UFOs and everything to do with taking over the world. If that's the case, their plan is working. Everybody is fixated on the alien connection. The state of Nevada even renamed the road running past Area 51 Extraterrestrial Highway. Talk about trivializing the fate of the human race!
Another possibility: The US Air Force really is just testing new aircraft in the desert. Yawn. That's too boring to be true.
The Clue hidden there is Calcium Carbonate.