Buried beneath its South Pole-Aitken basin – one of the largest preserved craters in the Solar System – is a structure which weighs at least 2.18 billion kilogrammes and measures more than 300km (186 miles) in depth and 2,000km (1,243 miles) in length.
The researchers who made the discovery, all based in the US, posited that the “anomaly” could be made out of metal from the core of an asteroid or oxides from the crystallisation of a magma ocean.
“One of the explanations of this extra mass is that the metal from the asteroid that formed this crater is still embedded in the Moon’s mantle,” lead author Peter B. James, from Houston’s Baylor University, said in a statement shared with IFLScience.
Illustrating just how gigantic this thing is, he went on: “Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground. That’s roughly how much unexpected mass we detected.”