New recordings have surfaced from NASA's 1969 Apollo 10 Moon mission...and on them you hear the crew reacting to strange "music" coming from an unknown source.
The crew was on the dark side of the moon when they heard the strange sounds. They weren't coming from Earth because they had no radio contact for about an hour.
Afterwards, they didn't talk about it publicly...
The transcripts were classified and untouched in NASA's archives until 2008.
So far, there's no official explanation as to what the crew heard.