The Wow! Signal

On August 15, 1977, Ohio State University's Big Ear radio telescope detected a 72-second narrowband radio signal from the direction of the constellation Sagittarius that was 30 times stronger than background noise. Astronomer Jerry Ehman circled the printout and wrote "Wow!" in the margin. The signal has never repeated despite hundreds of subsequent observations of the same region. It matched the expected signature of an extraterrestrial transmission on the hydrogen line (1420 MHz). A 2017 study proposed a comet as the source, but this was disputed. If it was natural, we've never found the mechanism. If it wasn't natural, we've never heard from them again.