In 1948, the US government launched several inquiries into UFO sightings, which many believed were sparked by Cold War paranoia.

In what became known as 'Project Blue Book,' more than 12,000 encounters with UFOs were looked into by the Air Force.

Now, decades after the files were closed, the microfilms have been made available online for free – allowing anyone to re-examine the evidence.

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Pictured is the staff of Project Blue Book, which recorded more than 12,000 encounters with UFOs. Sitting in the centre is Hector Quintanilla, the last chief officer of Project Blue Book

The USAF says that the Blue Book included 12,618 sightings reports, with 701 of which remained 'unidentified' – or around 5.5 per cent of the files.

The files, which can be accessed here, were previously only available by visiting the National Archives in Washington, according to a report in openminds.tv.

'There is plenty of work for amateur investigators to try to come up with explanations they never had time to consider or research,' Nigel Watson author of the UFO investigations Manual told MailOnline.

One case that Mr Watson is particularly intrigued in for historical reasons is a report filed by Kenneth Arnold which led to the popular term 'flying saucers'.

Mr Arnold saw nine UFOs over Mount Rainier, on 24 June 1947 and the US government agencies took an interest in reports. A similar sighting (pictured) occurred over Tulsa, Oklahoma on July 12, 1947

The information was transferred online after John Greenewald, who runs a website called The Black Vault, submitted thousands of Freedom of Information Act (FOIA) requests

WHAT IS PROJECT BLUE BOOK? Project Blue Book was the name for a project that investigated UFO reports between 1947 and 1969. It was the third study of its kind. The first two were projects Sign (1947) and Grudge (1949). The aim was to determine if UFOs were a threat to national security, and to scientifically analyse UFO-related data. The USAF says that Blue Book included 12,618 sightings reports, with 701 of which remained 'unidentified' – or around 5.5 per cent of the files. A termination order was given for the study in December 1969, and all activity under its project ended in January 1970. Advertisement

Mr Arnold saw nine UFOs over Mount Rainier, on 24 June 1947 and the US government agencies took an interest in reports.

A similar sighting occurred over Tulsa, Oklahoma, just a month later on July 12, 1947

'They flew in a diagonal formation that stretched about five miles from the first to last craft, and they bobbed about erratically,' said Mr Watson.

'He estimated that they travelled faster than 1,000 miles per hour as they flew from Mount Rainier to Mount Adams.'

When describing the sight to newspaper reporters, Mr Arnold said it was 'like a saucer would if you skipped it across the water.'

This, said Mr Watson, reveals that the term flying saucer came from the description of the movement of the craft rather than their appearance.

'Explanations from mirages, secret aircraft to the flight of pelicans have all been used to try to explain his sighting but it still remains a mystery,' he added.