The US-based Central Intelligence Agency (CIA) has published over 11 million pages of recently-declassified documents.
It says this happens automatically to non-exempt records 25 years or older under Executive Order 13526.
December 31st 2006 was the first major deadline for automatic declassification.
Since 2000, CIA has installed and maintained an electronic full-text searchable system, named CREST.
The system is a publicly accessible repository of records reviewed under the 25-year program.
More than 11 million pages have been released, from which researchers have printed about 1.1 million pages.
They include a report on Unidentified Flying Object (UFO) sightings, which says: "The IAC has review the current situation concerning unidentified flying objects which have caused extensive speculation in the press and have the subject of concern to Government organisations".
The undated document adds: "Since 1947, approximately 1,500 official reports of sightings have been received, of these about 20% are as yet unexplained".
Image: CIA
"It is therefore recommended that the Department of Defense (be directed to) undertake an expanded scientific research programme to reveal the nature of the various phenomena", it adds.
Other documents reveal details on a plan for a Berlin Tunnel in 1952, which was scrapped in 1956.
It would have seen a massive tunnel running to intercept Soviet and East German communications.
A transcript of a CIA meeting between six people on October 30th, 1979 details how the agency wanted to 'keep track of the Russians'.
"I have a feeling that we are going to have a difficult time keeping anything quiet long enough to do anything", a person identified as LTG Tighe says.
Image: CIA
A recipe for invisible ink is also included in the tranche.
"Make a silver print, fixed and bleached in mercury chloride. To make visible, dip in hypo", it says.
They have also revealed documents from the so-called Stargate programme, which dealt with psychic powers and extrasensory perception.
A document entitled "Magician Walks Into The Laboratory" looks at stories from Tbilisi, Georgia about a man who apparently heals people with just his hands.
"It would be well to register (for example, photograph) this strange phenomenon", it says.
It also contains profiles on several aspects of different countries - such as transport and military geography.
The profiles are on countries including Malta, Sweden, Germany, Poland, Yemen and South Korea.
A document on Ireland seems to list over 100 pages of place names, and gives their coordinates on a map.
