The Wow! Signal (1977)

On August 15, 1977, Ohio State University's Big Ear radio telescope detected a powerful narrowband radio signal from the direction of the constellation Sagittarius. Astronomer Jerry Ehman circled the intensity reading '6EQUJ5' on the printout and wrote 'Wow!' — giving it its name. The signal lasted exactly 72 seconds (the full observation window), appeared at the hydrogen frequency (1420 MHz, the frequency SETI considers most likely for interstellar communication), and was 30 times louder than background noise. Despite decades of follow-up searches — including Breakthrough Listen's targeted search in 2022 using the Green Bank Telescope and Allen Telescope Array — the signal has never recurred. Every proposed natural explanation (comets, space debris, interstellar scintillation) has been challenged or disproven. The signal remains unexplained. It matched every criterion scientists had established for what an extraterrestrial signal should look like — and then never repeated.