A recording of strange “music” heard by three NASA astronauts after they reached the far side of the Moon in 1969 has been made public for the first time.
The peculiar whistling noises were recorded and sent back to mission control in Houston, Texas.
Thomas Stafford, John Young and Eugene Cernan were aboard Apollo 10 at the time, and a member of the trio can be heard in the recording comparing the sound to “outer-space-type music”.
The astronauts were reluctant to tell NASA bosses about the noises for fear they would not be taken seriously, or even dropped from future space missions.
A NASA engineer has said radio interference was probably to blame for the sound - but not all astronauts are convinced.
Al Worden, who travelled on Apollo 15, said: “Logic tells me that if there was something recorded on there, then there’s something there.”
Michael Collins, who flew Apollo 11 solo around the far side of the Moon while fellow astronauts Buzz Aldrin and Neil Armstrong became the first men to walk on the surface, said he too had heard an “eerie woo-woo sound” - but he had been told to expect it before his mission.
“Had I not been warned about it, it would have scared the hell out of me,” Mr Collins wrote in a book.
The Moon’s rotation is such that the same side is always facing observers on Earth, and the far side is out of the range of Earthly radio signals.
The audio was unearthed for a Discovery TV series called NASA’s Unexpected Files.