A 72-second narrowband radio signal received by Ohio State's Big Ear telescope in 1977. Astronomer Jerry Ehman circled the printout and wrote 'Wow!' in the margin. Never detected again.
From Wikipedia
The Wow! signal was a strong narrowband radio signal detected on August 15, 1977, by Ohio State University's Big Ear radio telescope in the United States, then used to support the search for extraterrestrial intelligence. The signal appeared to come from the direction of the constellation Sagittarius and bore expected hallmarks of extraterrestrial origin.