He noticed a series of letters and numbers that represented a crescendo in the surrounding static. He circled the sequence and wrote an exclamation next to what became known as the “Wow!” signal.

The signal’s bandwidth was off-limits to human broadcasts. Even weirder, the frequency was about 1420 megahertz, the same as that emitted by hydrogen, the most common element in the universe. As such, scientists have reasoned that alien civilizations might use it to communicate.

The possible origin was narrowed to somewhere in the constellation Sagittarius, but despite many searches the noise was never heard again.

What was it? Theories include a military broadcast, a sound deflected off orbiting space junk, a malfunction, a yet-to-be-understood space phenomenon — and aliens. Scientists sent a reply to the signal in 2012, but it has — as yet — gone unanswered.

Penn Bullock contributed reporting.

_____

This briefing was prepared for the European morning. You can browse through past briefings here.

We also have briefings timed for the Australian, Asian and American mornings. You can sign up for these and other Times newsletters here.

Your Morning Briefing is published weekday mornings and updated online.

What would you like to see here? Contact us at europebriefing@nytimes.com.