In 1977, someone — or something — sent a radio signal that has never been explained

In August 1977, the Big Ear radio telescope in Ohio, listening for signals as part of the SETI program, picked up a strong, narrowband radio signal lasting 72 seconds — unusual enough that astronomer Jerry R. Ehman, scanning the printout, scrawled "Wow!" next to it by hand.

The signal had characteristics that matched theoretical expectations for a potential extraterrestrial transmission — a specific, narrow frequency close to the so-called hydrogen line, considered a "natural channel" for interstellar communication.

The twist: despite repeated attempts, that exact signal has never been detected again from the same patch of sky — not by Big Ear, and not by any later, more sensitive instrument.

Its source remains just one hypothesis among several: everything from a passing comet (a theory proposed decades later and partly disputed) to a mundane, terrestrial interference source — none confirmed, which makes "Wow!" one of the longest-standing open questions in the search for alien life.