The astronauts of Apollo 10 may have been overshadowed by the crew of Apollo 11, which was the first crew to land on the lunar surface. They do own another overlooked distinction, however: They were the humans to have traveled to the furthest point away from home in history. And it's what they heard while drifting out there beyond the far side of the moon that's just now starting to come into the limelight.

Apparently, during a period of time when the astronauts were shielded behind the moon, they recorded strange musical sounds coming through the Apollo module's radio, reports The Huffington Post.

Newly released transcripts that were kept as classified in NASA's archives until just 2008 reveal some of the astonishment the astronauts had while hearing the sounds:

"It sounds like, you know, outer space-type music."

"You hear that? That whistling sound? Whooooooooo!"

"Well, that sure is weird music!"

The sounds are particularly jarring because they were recorded while the spacecraft traversed the far side of the moon, out of radio contact with Earth. In other words, the music couldn't have been coming from home. The astronauts were so shocked that they even considered withholding the information from Mission Control.

"If you're behind the moon and hear some weird noise on your radio, and you know you're blocked from the Earth, then what could you possibly think?" said Apollo 15 astronaut Al Worden, who was also interviewed on the subject for the Science Channel series, "NASA's Unexplained Files."

The origin and nature of the strange space music is still a source of debate today, though there are some who have theorized that the radios in the spacecraft may have been interfering with each other, thus causing the spooky hubbub. Others are skeptical of this explanation, however, since this kind of disturbance would have been fairly easy to identify.

"The Apollo 10 crew was very used to the kind of noise that they should be hearing. Logic tells me that if there was something recorded on there, then there was something there," said Worden.

No other spacecraft to traverse the far side of the moon has ever recorded similar sounds — at least, none that are known from recordings that have been unclassified. So the mystery remains. You can see part of the Science Channel's coverage of this story below, and even hear some of the weird music: