A huge mysterious blob has been discovered lurking far beneath the solar system's largest crater, a vast depression on the moon known as the South Pole-Aitkin basin.

Scientists aren't sure what the blob is, but they think it could be a colossal mass of metal left over from the asteroid that created the crater billions of years ago when it smashed into the lunar surface.

Whatever it is, it's big: the scientists who found the blob estimate its mass to be at least 2.18 quintillion kilograms (about 4.8 quintillion pounds on Earth). That's roughly 6 billion Empire State Buildings — or five times more than the Big Island of Hawaii.

"This is a very large mass of a scale that is difficult even for geologists to recognize," said Paul Byrne, an assistant professor of planetary geology at North Carolina State University in Raleigh and a member of the team of scientists who made the discovery.

The researchers made the discovery with help from data from a trio of moon-orbiting NASA spacecraft. They compared gravitational maps of the moon made by the pair of washing-machine-sized Grail spacecraft that orbited the moon in 2012 with topographic maps of the lunar surface obtained by the Lunar Reconnaissance Orbiter, which has been circling the moon since 2009.

As the scientists say in a paper published April 5 in the journal Geophysical Research Letters, they noticed a "large excess of mass" more than 100 miles beneath the basin — a vast, oval-shaped depression on the lunar far side as wide as 1,550 miles (2,500 kilometers) with a depth of more than 5 miles (8 kilometers).

The finding indicated the presence of something massive beneath the surface.

Byrne said the mass concentration, or mascon, was likely a metal-rich remnant of the asteroid that struck the moon at least 4 billion years ago. "It's the best explanation we have with the data we have," he said.

Another possibility, Byrne said, is that it's a blob of metal-rich lunar minerals that melted when the asteroid struck and then sank beneath the surface and resolidified. Or it could be metal-rich minerals from the ocean of molten rock that's believed to have covered the moon in its early days before it cooled and hardened to form the surface we see today.

Scientists are eager to learn more about the object, as it could hold clues about the formation of Earth as well as of the moon.

"It lets us see a time when we have nothing [similar] preserved on Earth," said Walter Kiefer, a senior staff scientist at the Lunar and Planetary Institute in Houston who was not involved in the new research. "Everything on the Earth is younger than that because of all the geology" that has taken place on our planet in the billions of years since it formed — a reference to tectonic activity, volcanic eruptions and other processes that have erased evidence of similar impact craters on our home planet.

Byrne said future missions to the moon could explore the object in greater detail and discounted any notions that the metal within the blob would be mined. "It's too deep," he said, adding that if earthlings were looking for an off-planet source of metal, "it would be much easier to go and mine asteroids."

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