About 13 million pages of declassified documents from the US Central Intelligence Agency (CIA) have been released online.
The records include UFO sightings and psychic experiments from the Stargate programme, which has long been of interest to conspiracy theorists.
The move came after lengthy efforts from freedom of information advocates and a lawsuit against the CIA.
The full archive is made up of almost 800,000 files.
They had previously only been accessible at the National Archives in Maryland.
The trove includes the papers of Henry Kissinger, who served as secretary of state under presidents Richard Nixon and Gerald Ford, as well as several hundred thousand pages of intelligence analysis and science research and development.
Among the more unusual records are documents from the Stargate Project, which dealt with psychic powers and extrasensory perception.
Those include records of testing on celebrity psychic Uri Geller in 1973, when he was already a well-established performer.