Ufologists can now pore over the details of 12,618 'alien' encounters documented and investigated by the US Air Force between 1948 and 1970
Ufologists can now pore over the details of 12,618 'alien' encounters documented and investigated by the US Air Force between 1948 and 1970.
The UFO probe became known as Project Blue Book.
Of the 12,618 sightings, 701 (around 5.5 %) remain unidentified.
The Blue Book files were previously only available by visiting the National Archives in Washington.
Nigel Watson, author of the UFO Investigations Manual, told Mail Online: "There is plenty of work for amateur investigators to try to come up with explanations they never had time to consider or research."
He said one particularly intriguing report was the sighting by Kenneth Arnold of nine UFOs over Mount Rainier, in Washington state, on June 24, 1997.
It was this sighting which led to the use of the term "flying saucers" to describe UFOs.
A similar sighting then took place over Tulsa, Oklahoma, three weeks later.
When describing the sight to newspaper reporters, Mr Arnold said it was "like a saucer would if you skipped it across the water".