Researchers have discovered something massive lurking underneath the far side of the moon: a mysterious blob with the mass akin to a pile of metal five times the size of the Big Island of Hawaii.

The structure, described in a recent study published in Geophysical Research Letters, sits at least 180 miles beneath the South Pole-Aitken basin—a colossal crater punched into the lunar landscape billions of years ago, when the moon's initially molten surface had cooled just enough for impacts to leave a lasting mark.

The team discovered the anomalous blob by combining data from NASA's Gravity Recovery and Interior Laboratory, or GRAIL, mission with topography from the Lunar Reconnaissance Orbiter. This data helped them refine past calculations for the thickness of the crater's crust and the density of the mantle, revealing the odd underground excess of mass.

The blob is likely related to the crater's formation, and it may be the remnants of an ancient impactor's metal core, says study coauthor Peter James of Baylor University. While the excess mass isn't immediately obvious from the surface, it does seem to be having quite an effect, dragging down the lunar landscape in a curious ovoid depression that sits more than half a mile lower than the surrounding crater floor, a feature known as the central depression.

“That’s a huge result,” says Daniel Moriarty, a lunar geologist with NASA's Goddard Space Flight Center. “It really gives us a hint of what’s going on in the lunar interior.”

View Images This false-color image shows the topography on the far side of the moon, with highs in warm colors and lows in cool colors. A dashed circle indicates the zone of excess mass under the South Pole-Aitken basin. Image by NASA/Goddard Space Flight Center/University of Arizona

In the past, the South Pole-Aitken crater has garnered plenty of interest both for its surface composition and its size. (Learn more about strange rocks found in the South Pole-Aitken basin that may have come from deep inside the moon.)

“It’s the biggest preserved crater that we know of in the solar system,” James says. The discovery of the odd mass only adds to the intrigue, especially since the crater and the nearby lunar south pole are potential targets for multiple future missions to the moon.

Scientists are already raring to study the mass. Such an effort could help unravel the history behind the monumental impact that created the crater—and fill in crucial details in our understanding of how our lunar companion and other celestial bodies grow over time.

“As an impact modeler, it’s very exciting,” says Brandon Johnson, a planetary scientist at Brown University who was not involved in the new study. “I can’t wait to possibly get started working on this.”

A massive discovery

The GRAIL mission's duo of spacecraft—dubbed Ebb and Flow—launched in 2011 and orbited the moon for nearly a year, precisely charting variations in the lunar gravitational field. Using this data, the GRAIL team constructed the highest-resolution gravity map of our lunar companion yet.

Moon 101 What is the moon made of, and how did it form? Learn about the moon's violent origins, how its phases shaped the earliest calendars, and how humans first explored Earth's only natural satellite half a century ago.

The data give a loose picture of what's happening both on the surface and underground. The more mass there is, like higher topography or denser rocks, the stronger the gravity. These maps highlight a striking difference between most of the moon's large craters and the South Pole-Aitken basin.

Other large craters have what are known as mascons, short for mass concentrations. Discovered in 1968 by scientists at NASA's Jet Propulsion Laboratory, mascons show up in gravity maps as bullseyes—a central circle of strong gravity surrounded by a ring of weak gravity and then another ring of stronger gravity. The phenomenon is a consequence of the way low-density crust and high-density mantle adjust after an impact.

But the South Pole-Aitken basin doesn't have such a pattern. So to figure out what was going on under the surface, the scientists turned to calculations, creating a model using new assumptions about the forces at play that more accurately reflects the natural system. The result revealed the large zone of dense material that sits within the moon's upper mantle.

Core relic?

The team suggests two possibilities to explain the subsurface mass. First, it could be remnants of dense oxides that formed in the final stages of cooling back when the moon was covered in ancient magma oceans. But the researchers don't have a mechanism to precisely explain the formation of such a layer specifically under the basin.

“Why would it be there, of all places?” James asks.

Instead, the mass could come from an ancient impactor, the team argues. The space rock that formed the moon's giant basin was likely large enough to have separated into different layers when it first formed, so that like many of today's planets, it sported a dense, metallic core and rocky outer layers.

On the fateful day of its collision, the energy of the impact carved a deep bowl-shaped crater on the moon, with the impactor's metallic core smashed up inside. But the original hole didn't last, and the divot on the moon partially refilled with molten rock. Within it lingered the melty traces of the ancient impactor's core.

“That's what I would bet on,” James says.

“It’s really convincing that there’s something there,” Johnson says, agreeing that a core relic is a likely explanation. “The whole time I was reading [the study], I was thinking about all the different ways we can follow up and try to better understand what is causing this mass anomaly that they've found."

Deepening curiosity

In addition to spotting the mysterious blob, the new study retraced the boundary of the basin's inner rim, revealing that scientists previously underestimated the crater's size, a potentially important find as NASA and others prepare to send missions to the basin and the nearby lunar south pole. The last researchers to map out these limits used data from the Clementine mission, which had a gap near the basin's southern extent. The latest work, however, used more complete data from LRO and GRAIL, which revealed that the crater is roughly 40 miles larger than once thought.

Overall, the work deepens the curiosity already surrounding the South Pole-Aitken basin.

“It's just so mysterious,” says Sara Mazrouei of Western University's Center for Planetary Science and Exploration, who was not involved in the work. And by improving our understanding of this structure, scientists hope to better understand the formation of bodies throughout our celestial family.

“Every planet in our solar system was formed by little things smacking into each other and eventually forming bigger things,” Moriarty says.

On Earth, the perpetual churn of plate tectonics has been steadily erasing the planet's ancient surface and its record of early impacts. But the moon, still sporting a surface that's billions of years old, serves as an incredible record of what happened when our solar system was merely an infant—including the dramatic events that formed one of the largest known impact basins in our cosmic neighborhood. (Find out why scientists now think the moon may still have some tectonic activity.)