Scientists have found a nearly square peg underneath a round hole—on the moon. Several kilometers below Oceanus Procellarum, the largest dark spot on the moon’s near side, scientists have discovered a giant rectangle thought to be the remnants of a geological plumbing system that spilled lava across the moon about 3.5 billion years ago. The features are similar to rift valleys on Earth—regions where the crust is cooling, contracting, and ripping apart. Their existence shows that the moon, early in its history, experienced tectonic and volcanic activity normally associated with much bigger planets.

“We’re realizing that the early moon was a much more dynamic place than we thought,” says Jeffrey Andrews-Hanna, a planetary scientist at the Colorado School of Mines in Golden and lead author of a new study of the Procellarum’s geology. The discovery also casts doubt on the decades-old theory that the circular Procellarum region is a basin, or giant crater, created when a large asteroid slammed into the moon. “We don’t expect a basin rim to have corners,” Andrews-Hanna says.

The work is based on data gathered by GRAIL (Gravity Recovery and Interior Laboratory), a pair of NASA spacecraft that orbited the moon in 2012. Sensitive to tiny variations in the gravitational tug of the moon, GRAIL mapped density variations below the surface (because regions of higher density produce slightly higher gravitational forces). Below known impact basins, GRAIL found the expected ringlike patterns, but underneath the Procellarum region, the mysterious rectangle emerged. “It was a striking pattern that demanded an explanation,” Andrews-Hanna says.

Scientists already know that the Procellarum region is rich in radioactive elements that billions of years ago would have produced excess heat. The study team theorizes that as this region cooled, the rock would have cracked in geometrical patterns, like honeycomb patterns seen on Earth in basalt formations, but on a much larger scale. In a study published today in Nature, the researchers propose that these cracks eventually grew into rift valleys, where magma from the moon’s mantle welled up and pushed apart blocks of crust. Lava spilled out and paved over the Oceanus Procellarum, creating the dark spot that is seen today. The extra weight of this dense material would have caused the whole region to sink slightly and form the topographic low that has made the Procellarum seem like a basin.

With the discovery, the moon joins Earth, Mars, and Venus as solar system bodies with mapped examples of rifting. There are also similar features near the south pole of Enceladus, the moon of Saturn that is spewing water into space from cracks in an ice shell.

Andrews-Hanna and colleagues have made a good case, says Herbert Frey, a planetary scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, even though the newly described features are surprising. The moon is not big enough to have the same strong convective cooling process that Earth has in its interior, he explains, and ordinarily convection is one of the main mechanisms thought to lead to large-scale rifting. So just what caused the rifting remains unclear. “It just means the moon continues to surprise us,” he says. Frey adds that a remaining mystery is why the rectangular features were found only beneath Oceanus Procellarum. Even if the rifting is explained by the excess radioactive elements, there is still no definitive explanation for why only the near side of the moon ended up enriched.

The discovery could also be a death knell for the impact theory for Oceanus Procellarum, an idea first put forth in the early 1970s. A basin there would have been the largest on the moon—larger than the South Pole–Aitken Basin—and second in the solar system only to the Borealis Basin on Mars, which covers the planet’s entire northern hemisphere.

Ryosuke Nakamura, a researcher at the National Institute of Advanced Industrial Science and Technology in Tsukuba, Japan, is still not convinced that an impact can be ruled out. In 2012, he and his colleagues published a paper in Nature Geoscience that found compositional evidence for an impact within Procellarum—a type of pyroxene mineral that is found in other, known impact basins such as South Pole–Aitken and is associated with the melting or excavation of mantle rock from an asteroid impact.

In response to the current study, Nakamura says that the features in the southwestern corner of the Procellarum region look to be circular rather than rectangular, and still consistent with an impact. But Frey, who has long been skeptical of the impact theory, says that the features are as clear as day, and not what you’d expect underneath a basin. “That looks like a rectangle to me.”