Although it looks like a boring, barren place, the Moon is home to some pretty amazing landmarks. The South Pole-Aitken basin, for example, is the largest impact crater in the solar system, measuring about 2,500 km (1,550 mi) across at its widest point. And now scientists have detected something strange buried under the crater.

The discovery was made by researchers studying differences in the strength of gravity in different parts of the Moon. Those measurements were made by NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission, and combined with data on the Moon's topography taken by the Lunar Reconnaissance Orbiter (LRO). That data revealed a huge mass of material buried several hundred miles below the surface of the basin.

"Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground," says Peter James, lead author of the study. "That's roughly how much unexpected mass we detected."

So what's down there? The team says that the crater itself is basically a four billion-year-old smoking gun, and the material was most likely deposited there by the object that left the scar on the lunar surface. Computer simulations showed that if the impactor was a large asteroid with an iron-nickel core, that metal may still be intact after all these years.

"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," says James. "We did the math and showed that a sufficiently dispersed core of the asteroid that made the impact could remain suspended in the Moon's mantle until the present day, rather than sinking to the Moon's core."

Of course this isn't the only possible explanation. Another suggestion is that the mass may be a dense lump of oxides, left over from when an ocean of magma was solidifying.

The answer could be revealed through further study of the South Pole-Aitken basin by GRAIL, the LRO, and China's Chang'e-4 lander, which touched down in the crater early this year.

The research was published in the journal Geophysical Research Letters.