A mysterious large mass discovered beneath the largest crater in our solar system — the Moon’s South Pole-Aitken basin — may contain nickel-iron deposit from an ancient asteroid collision, according to new research.

The crater, which is hidden from view on the far side of moon, is about 2000km wide and several kilometres deep.

“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,” said Peter B. James, lead author of Deep Structure of the Lunar South Pole-Aitken Basin.

The large mass was discovered when researchers analysed NASA data to measure subtle changes in the strength of gravity around the Moon.

“When we combined that with lunar topography data from the Lunar Reconnaissance Orbiter, we discovered the unexpectedly large amount of mass hundreds of miles underneath the South Pole-Aitken basin,” James said.

“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.”

The dense mass — “whatever it is, wherever it came from” — is weighing the basin floor downward by more than half a mile, he said.