For decades, the government went to great length to conceal any evidence of what actually had been built inside Area 51, even blocking release of a 1974 orbital photo of the area inadvertently taken by Skylab astronauts [source: Day].
The paucity of verifiable information about Area 51's architecture has fueled the feverish imaginations of conspiracy theorists. Some have suggested that the base has fake "scoot-and-hide" buildings, designed so that aircraft can be slid underneath them and hidden when a satellite is scheduled to pass overhead. Others have claimed that most of the base itself actually is hidden from view, and that a massive underground facility with as many as 40 levels lies beneath the surface [sources: Darkgovernment.com, Levy]
That subterranean infrastructure supposedly is protected at ground level by layers of security measures, ranging from sensors that can detect both the motion and scent of intruders, to black helicopters equipped with stealth technology [source: McConnachie and Tudge]. The underground hangars that contained captured UFOs, according to Lazar, were actually carved into the base of a mountain. Each had an entrance with a giant door with a simulated sand texture to blend into the landscape [source: Maloney].