When NASA astronauts circled the moon on the 1969 Apollo 10 mission—the final "test run" mission before the first moon landing later that year—they heard strange, whistling, musical noises while their spacecraft flew around the far side (often called the dark side) of the moon. Since there's a Moon between any spacecraft and Earth during this portion of the flight, Apollo was receiving no signals from anywhere but space. Now, after more than 40 years, the Discovery Science show NASA's Unexplained Files uncovered the recording of that "music," which was buried deep in NASA's lengthy historical archives (though technically available to the public). Take a listen: