A “mysterious” gravitational mass has been discovered on the far side of the moon buried below a 4-billion-year-old crater.

In research published in the journal Geophysical Research Letters, scientists say the mass may contain metal from the asteroid that created the South Pole-Aitken basin. In January, China landed a probe for the first time in the crater, which is the largest in the solar system at some 2,000 km wide, roughly the distance between Victoria, B.C., and Brandon, Man.

The unidentified buried mass is huge. “Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground,” compared lead author Peter B. James, Ph.D., assistant professor of planetary geophysics at Baylor University, in a school news release Monday.

Using data from NASA spacecraft, researchers measured the strength of gravity around the moon and observed an “unexpectedly large amount of mass” hundreds of kilometres under the crater. “Whatever it is, wherever it came from,” as researchers put it, the mass is weighing the basin floor downward by more than half a mile, or 0.8 km.

According to their calculations and simulations, an asteroid with an iron-nickel core could have dispersed across the moon’s upper mantle and remained there until present day, rather than sinking to the core.

But the asteroid theory is just one guess they have to explain the anomaly. Another is that the mass is a concentration of gases from the last stage of solidification of the molten ocean after the formation of the moon.