In 1977, a mysterious 72-second radio signal from deep space left an astronomer so stunned he scrawled "Wow!" on the printout — and it's never been explained.
On August 15, 1977, Ohio State University's Big Ear radio telescope detected a powerful narrowband radio signal appearing to come from the constellation Sagittarius. The signal bore all the expected hallmarks of extraterrestrial origin, and it has never been satisfactorily explained.
Astronomer Jerry R. Ehman discovered the anomaly days later while reviewing printouts by hand. He circled the intensity reading "6EQUJ5" and scrawled "Wow!" in the margin — giving the mysterious detection its famous name.
The signal's peak intensity, represented by the letter "U," was an astonishing 30 standard deviations above background noise. Its frequency of approximately 1420 MHz sits right on the hydrogen line — the exact frequency scientists had predicted an intelligent civilization might use to communicate.
The signal lasted exactly 72 seconds, which was precisely how long the Big Ear telescope could observe any fixed point in the sky due to Earth's rotation. Its gradual rise and fall in intensity perfectly matched what a continuous extraterrestrial signal would look like.
Despite over 50 follow-up searches using increasingly powerful telescopes — including the Very Large Array and equipment in Tasmania — the signal has never been detected again. This lack of replication remains the biggest obstacle to confirming its origin.
In 2017, astronomer Antonio Paris proposed that hydrogen clouds around two comets caused the signal, but this was swiftly dismissed by experts. The comets weren't in the beam at the right time, don't emit strongly at the relevant frequencies, and couldn't explain the signal appearing in only one feed horn.
In August 2024, the Planetary Habitability Laboratory published findings from Arecibo Observatory suggesting the signal was likely caused by a rare astrophysical event — stellar emissions energizing a cold hydrogen cloud, causing it to suddenly surge in brightness.
In 2022, researchers identified three likely Sun-like stars within the signal's coordinates, including one 1,800 light-years away. Breakthrough Listen then conducted the first targeted search using the Green Bank Telescope and Allen Telescope Array simultaneously, but found no technosignatures.
In 2012, the Arecibo Observatory beamed roughly 10,000 Twitter messages toward three stars as a symbolic reply to the Wow! signal. However, the transmitter overheated during the first transmission, and ironically, none of the targeted stars were even in the region the signal came from.