Area 51 has been synonymous with alien life conspiracies and military cover-up theories, although the U.S. Air Force has maintained it is a flight testing facility connected with Edwards Air Force base in California. This satellite image shows the remote, secret military installation in the Nevada desert.

Tensions between the U.S. and USSR were rising as the Cold War began after World War II. Area 51 was opened in 1955 to test the newly developed Lockheed U-2 spy plane, shown here. It was a cutting-edge reconnaissance aircraft, flying so high it could evade antiaircraft fire and show up minimally on Soviet radar.

Today, Area 51 remains a highly restricted area. This photo shows the base's back gate, also known as the north gate, located near the town of Rachel, Nev.

Remote controlled cameras at the north gate.

Groom Lake Road is the main road leading into Area 51 from the north. In this photo, a security vehicle (circled in red) can be seen on a side road on the hill.

This photo of Groom Lake Road shows the view away from the north gate, back towards Hwy. 375, which runs along the base of the mountain range in the background. The mountain on the right is Tikaboo Peak. Located 26 miles from the north gate, it is the closest visitors are allowed to get to the restricted area to view the base.

Surveillance cameras near the Groom Lake Road entrance into the restricted area.

Large red and white caution signs along Groom Lake Road leading up to the restricted area warn visitors that they are approaching a military installation should not come any closer. They also warn that trespassing could lead to a $1,000 fine and up to six months in prison.

New warning signs for the Nellis Bombing and Gunnery Range were posted on top of old ones in 2008, saying, "It is unlawful to enter this area without permission of the Installation Commissioner."