'Off-limits' sign from Area 51
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Area 51 acknowledged by CIA in declassified U-2 spy plane documents

17 August 2013 Last updated at 16:36 BST

The Central Intelligence Agency has officially acknowledged the secret US test site known as Area 51, in a newly declassified internal history of the U-2 spy plane programme.

The document, obtained by George Washington University, describes the 1955 acquisition of the Nevada site for the testing of the secret spy plane.

It also explains the site's lingering association with UFOs and aliens.

Jane Bennett-Powell reports.