While the exact origin of the mysterious material which lies underneath the dark side of the Moon remains unclear, scientists theorise that it might be the remains of an asteroid that smashed into the lunar surface.

Scientists studying the lunar surface have discovered a large mass of material of unclear nature and origin buried deep beneath the South Pole–Aitken basin, a vast crater located on the far side of the Moon, The Independent reports.

"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, assistant professor of planetary geophysics in Baylor's College of Arts & Sciences and lead author of the study.

As James explained, the discovery was made when the scientists analysed data from spacecraft used for the NASA Gravity Recovery and Interior Laboratory (GRAIL) mission, and combined it with “lunar topography data from the Lunar Reconnaissance Orbiter”.
According to the newspaper, while it currently remains unknown where this material came from, researchers already theorise that it might be the remains of an asteroid which smashed into the celestial body’s surface.

The scientists note, however, that there’s also another possibility - the mass of material in question might be a “concentration of dense oxides associated with the last stage of lunar magma ocean solidification”, as Science Daily points out.