Back in 1971, Apollo 15 astronauts orbiting the Moon photographed something very odd. Researchers called it “Ina,” and it looked like the aftermath of a volcanic eruption.

There’s nothing odd about volcanoes on the Moon, per se. Much of the Moon’s ancient surface is covered with hardened lava. The main features of the “Man in the Moon,” in fact, are old basaltic flows deposited billions of years ago when the Moon was wracked by violent eruptions. The strange thing about Ina was its age.

Planetary scientists have long thought that lunar volcanism came to an end about a billion years ago, and little has changed since. Yet Ina looked remarkably fresh. For more than 30 years Ina remained a mystery, a “one-off oddity” that no one could explain.

Turns out, the mystery is bigger than anyone imagined. Using NASA’s Lunar Reconnaissance Orbiter, a team of researchers led by Sarah Braden of Arizona State University has found 70 landscapes similar to Ina. They call them “Irregular Mare Patches” or IMPs for short.