On the night of August 15, 1977, Ohio State University's Big Ear radio telescope, then used in the search for extraterrestrial intelligence, picked up a strong, narrow-band radio signal from the direction of the constellation Sagittarius.
Days later, volunteer astronomer Jerry R. Ehman spotted it while reading the computer printouts by hand. He circled the code '6EQUJ5', which showed the signal's strength, and wrote 'Wow!' beside it. The signal lasted the full 72 seconds that Big Ear could watch any one spot, and it was close to 1420 MHz, the natural frequency of hydrogen, where scientists had suggested aliens might choose to transmit.
Despite many follow-up searches, the signal has never been seen again. Ideas have included space debris, comets and rare natural bursts from hydrogen clouds, but none has been confirmed. Big Ear itself was demolished in 1998.