The CIA posted almost a million declassified files online this week, offering a fascinating record of unexplained UFO sightings and the agency’s secret operations.

Around 930,000 documents, totaling more than 12 million pages, have been posted online. Previously, the files were only available to the public at the National Archives in Maryland. Spanning early CIA history, the Cold War, Vietnam, and the U-2 reconnaissance aircraft, the archive also details a number of UFO incidents.

A ‘UFO’ search of the archive at Electronic Reading Room on the CIA’s website returns 1,738 items, the earliest of which is dated 1942. Strange sightings are described in files such as a 1966 file detailing an “unusual phenomenon” on the horizon near the border between Iran and the former Soviet Union.

“As we were approaching the Rudeshur beacon, located approximately 25 miles WSW of Mehrabad Airport, Teheran, we suddenly observed a brilliant white sphere approximating the coloration and intensity of full bright moon,” the report says. “The sphere appeared suddenly and at the first sighting was approximately three times the size of a full moon.” The source’s identity is redacted.

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A 1953 document describes UFO sightings in Spain, Greece, Algeria and the then French Morocco, detailing a number of incidents based on local newspaper reports. The subject of the file is “Military – unconventional aircraft.”

This is not the first time that the CIA has opened up its UFO files. Last year the agency posted a smaller trove of once-classified UFO documents online.

The documents also offer a rare insight into the CIA’s clandestine operations, such as a confidential document titled “Invisible photography and writing, sympathetic ink, etc.” The file describes a number of methods for producing invisible ink, such as writing with starch on linen. Another method involves a heavily diluted sulphate of copper, which can be used “in writing with a soft tooth pick between printed lines.”

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Since 1999, the CIA has regularly released historical declassified records to the National Archives Records Administration (NARA), in College Park, Maryland, but is now placing more and more files online.

“Access to this historically significant collection is no longer limited by geography. The American public can access these documents from the comfort of their homes,” said Joseph Lambert, the CIA Director of Information Management, in a statement.