There's something buried deep beneath the moon's surface. It's massive — about five times the size of Hawaii's Big Island — and it's having a significant impact on the moon's gravitational field.

The thingie lies in the heart of the South Pole-Aitken Basin. (Seriously, the best name scientists could come up with when publishing their discovery in the journal Geophysical Research Letters, is that it's a "large excess of mass.")

At about 1,600 miles wide, the basin is one of the biggest known impact craters in our solar system. And now it appears to be home to one of the biggest unknowns in our solar system.

The mass — "whatever it is, wherever it came from," study co-author Peter James of Baylor University notes in an accompanying press release — is weighing the basin floor downward by more than half a mile.

"Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground," James explains in the release.

"That's roughly how much unexpected mass we detected."

China's Chang'e-4 probe snapped this photo of a crater on the moon's far side. CNSA

Adding to this lunar riddle is that the massive lump is on the fabled far side of the moon — a barren expanse that always faces away from our planet. As such, it's a place that's long thwarted prying Earthling eyes. In fact, most of the dark side — so named because it's out of view, rather than lacking light — wasn't observed until the Soviet space probe Luna 3 had a gander at it in 1959.

But aside from space-based ogling, no craft has touched down on that crater-ridden expanse until this year's historic landing by the Chinese probe, Chang'e-4.

The Chang'e-4 took this photo of the moon's surface shortly after landing. CNSA

That probe managed to send home stunning images of the pock-marked surface, including some from the South Pole-Aitken Basin.

But no one said anything about a secret alien base — errr, large excess of mass.

Probably because whatever lurks there is an estimated 185 miles below the surface.

What we do know — thanks to newly analyzed data from NASA's moon-gawking Lunar Reconnaissance Orbiter, as well as the Gravity Recovery and Interior Laboratory — is that the deposit is causing a major gravitational anomaly. According to the researchers, the density of the basin was higher than average for the moon's surface.

And scientists being scientists, have theories.

"One of the explanations of this extra mass," James notes, "is that the metal from the asteroid that formed this crater is still embedded in the Moon's mantle."

"We did the maths 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."

Alternately, the metallic mass in the heart of the basin could be a vestige from the moon's volcanic days, when seas of lunar magma flowed and then hardened.

The good news is that China is still making tracks in the region, with its plucky robot Yutu2 rover. Maybe it will help unroll this fresh mystery that's wrapped in the very old enigma that is the dark side of the moon.

Or maybe, just maybe, that secret alien base isn't so secret any more.