As the oval-shaped crater can not be seen from Earth because of its location on the far side of the Moon, researchers analysed data from spacecrafts used for NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission.
The study's lead author, Dr Peter B James, says that the mass is just unbelievably huge: "Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground".
According to researchers, the anomaly has a mass of 2.18×10^18 kilograms and is buried 300 km under the Moon's surface.
It sits 180 miles beneath the South Pole-Aitken basin - one of the solar system's largest impact craters, and the moon's oldest, at over 4 billion years - a massive dent spanning some 1,550 miles on the far side of the moon.
With humans headed back to the Moon sooner rather than later, the crater could be an interesting location for further study, though NASA and other space-faring organizations already have plenty of scientific objectives on their plates.
That new hypothesis is based on data from NASA's Gravity Recovery and Interior Laboratory (GRAIL) and Lunar Reconnaissance Orbiter missions. All of that metal, and basically the entire area surrounding the mass and the crater, could tell them a lot about how the asteroid impact happened and what the solar system was like when it did. The crater is several kilometres deep but can not be seen from Earth because it is located in the far side of the Moon, which is permanently turned away from Earth for astronomers and telescopes to study. James is assistant professor of planetary geophysics at Baylor's College.More news: Trump calls French wine tariffs unfair, vows to 'do something' about it
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The gravitational tug of the Moon was mapped on the NASA's GRAIL mission, where two spaceships used one another to ping signals for more than a year.
The second theory is that the mass is a concentration of dense oxides leftover from the last stage of what scientists call the lunar magma ocean crystallization.
The typography of the Moon anywhere around the South Pole-Aitken basin is particularly noticeable as charted by the two lunar missions.
The Chinese lander Chang'e-4 and its Yutu-2 rover are now exploring the Von Karman crater within the South Pole-Aitken basin, and NASA also wants to target the South Pole for future exploration.
The researchers' simulations imply the material could be an asteroid's iron-nickel core, which, if dispersed into the upper mantle, could be weighing down the basin. While many bigger impacts have occurred in the solar system and even on Earth, evidence of those impacts have been lost.
So, some 4 billion years ago, a mostly metallic asteroid hit the moon and remains embedded in the mantle to this day.