On August 15, 1977, Ohio State University's Big Ear radio telescope detected a strong narrowband radio signal designated '6EQUJ5' that astronomer Jerry R. Ehman famously marked with 'Wow!' The signal appeared to come from Sagittarius and matched expected extraterrestrial characteristics. It lasted the full 72-second observation window and registered 30 standard deviations above background noise.

The signal's frequency of 1420.4556 MHz was slightly above the hydrogen line, suggesting possible blue-shift. Despite numerous follow-up searches and hypotheses including reflections from space debris or comet hydrogen clouds, the signal has never recurred and no explanation has been confirmed. Located near Perkins Observatory in Delaware, Ohio, the Big Ear telescope was only adjustable for altitude and relied on Earth's rotation, creating uncertainty about the precise signal origin.

While some researchers suggest it could be extraterrestrial, its single occurrence limits interpretation strength. The Wow! signal inspired targeted searches, scientific discussion, and cultural references, representing the longest-running SETI program in history.