Apollo 10 was the fourth manned mission in the Apollo program, and launched on May 18, 1969. Launched on May 18, 1969, it was the F mission: a “dress rehearsal” for the first Moon landing, testing all of the components and procedures, just short of actually landing. Though the Apollo 10 audio and transcripts were not officially classified, there was “no way to get them to the public before the Internet,” the NASA History Office clarified Monday morning on Twitter.

— NASA History Office (@NASAhistory) February 22, 2016 The NASA ground control could not hear the sounds as they were out of radio contact. A report by E.W. Scripps National Desk says that the Apollo 10 flight crew observed the sounds while on the dark side of the moon. Michael Collins, the astronaut on the next mission to the moon, Apollo 11, tried to give a scientific answer to this eerie sound phenomenon. In a book published years later, Collins said the noise was just interference between the lunar module and command module’s VHF radios. This theory was neither confirmed nor denied by NASA. Recordings of the strange sounds are featured on the Science Channel show “NASA’s Unexplained Files,” which will air on 23rd February at 10 p.m. ET.