In 1977, researchers at Ohio State picked up this narrow-band radio signal emanating from somewhere beyond the constellation Sagittarius. It was exactly the kind of signal you'd expect from an extraterrestrial intelligence. What was it?
The sound became known as the "wow signal" because the astronomer who first saw it in a readout circled the signal and wrote "wow!" next to it. Nobody knows for sure what caused it, though the sound was real, documented by the "Big Ear telescope" at Ohio State, an observatory that detects radio emissions.
This is just one of 8 "unsolved sounds" documented by the sleuths over at Web Urbanist. They include the "bloop sound" detected by several different underwater sonar sensors in 1997, well as this "slow down sound" (detected the same year as the "bloop"). Like the "bloop," it was detected deep underwater, by several different monitoring systems, and never heard again.
Then there's the weird music emitted by the rings of Saturn and the bizarre "boom" noises heard off the coast of several different continents. You must go read this entire, scary article and then speculate about what, exactly, caused all these noises.
via Web Urbanist