A large metal mass five times the size of Hawaii has been discovered under the moon's largest crater.

A large mass thought to be metal from an asteroid is weighing down the brightest and largest object in our night sky.

“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 lead author Dr Peter James, Assistant Professor of Planetary Geophysics at Baylor University in Texas.

Dr James, in an April 2019 study published in the journal Geophysical Research Letters, believes the mass created a crater, known as the Lunar South Pole-Aitken Basin.

At roughly 2,500 kilometres (1,550 miles) in diameter, the Lunar South Pole-Aitken basin covers approximately one-fourth of the moon’s surface, according to NASA. The circumference is roughly 11,000 km.

In addition to being the largest crater on the moon, the Pole-Aitken basin is also one of the largest known impact craters in the solar system and is thought to be approximately 4 billion years old.

James and his team looked at data from NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission and the changes in gravity they discovered surprised them.

“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.”

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The anomaly – “whatever it is, wherever it came from,” James added – is weighing down the basin floor by more than half a mile. The team of researchers ran computer simulations that show the iron-nickel core of an asteroid could have been placed into the upper mantle of the Moon after impact.

“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,” James noted.

One other possibility for the unexplained mass is that it could be an area where dense oxides compiled following the last stage of lunar magma ocean solidification.

Whatever it is, James said the basin is “one of the best natural laboratories for studying catastrophic impact events, an ancient process that shaped all of the rocky planets and moons we see today.”