'A huge, mysterious "mass of material" has been spotted underneath the Moon.
The vast chunk is sitting underneath the biggest crater in the entire solar system, known as the South Pole-Aitken basin.
And it may be part of the material that smashed into our neighbour and formed the planet, according to the researchers who discovered it. But that is just one theory on where it may have come from, and scientists stress that it is still unknown what the material is and where it might have come from.
"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 on a new study of the huge chunk.
The crater that the mass sits on top of is over-shaped and 2,000 kilometers wide. It goes down several miles.
Though it is incredibly vast, it can't be seen from Earth because it is on the so-called dark side of the Moon, which faces away from us at all times.
Instead, it was examined by data to sent to researchers by Nasa spacecraft that sent back data on the small changes in 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," said Professor James, whose research has been published in Geophysical Research Letters. "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."
Scientists simulated a possible collision with an asteroid on a computer, and found that some kinds of metal could settle into the layer between the crust and the core, where this chunk is still sitting.'
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