The CIA is acknowledging for the first time the existence of Area 51 in newly declassified documents.
The mysterious piece of land in Nevada has been linked with reports of UFOs, alien bodies and secret government projects.
Now George Washington University's National Security Archive has obtained a CIA history of the U-2 spy plane programme through a public records request and released it.
National Security Archive senior fellow Jeffrey Richelson reviewed the history in 2002, but all mentions of Area 51 had been redacted.
Mr Richelson says he requested the history again in 2005 and received a version a few weeks ago with mentions of Area 51 restored.
Officials have already acknowledged in passing the existence of the facility in central Nevada where the government is believed to test intelligence tools and weapons.
Mr Richelson believes the new document shows the CIA is becoming less secretive about Area 51's existence, if not about what goes on there.