There's a strange bulge on the Solar System's largest moon, one that measures 375 miles wide and nearly two miles tall. Scientists aren't entirely sure why it's there or what caused it, but it may have something to do with the Jovian moon's subsurface ocean.

As reported by Nadia Drake in National Geographic, the protuberance is about the size of Ecuador and half the height of Mount Kilimanjaro.

Related: There May Be More Water On Jupiter's Largest Moon Than On Earth

This equatorial bulge, possibly the result of "polar wandering", could mean the moon's poles have shifted by 90 degrees, and that the moon truly has a subterranean ocean. Drake writes:

...Schenk thinks, the bulge began growing at one of the poles. Then, once the bulge grew big enough, its mass began to drag the shell into a different position. The shell slid atop the ocean, while the moon's interior stayed in the same orientation. Eventually, the part of the shell that once capped the poles ended up at the equator.

"The only place you can get a large mass like that—that's not related to geology that we know of—is at the cold poles," Schenk says. "The poles are permanently cold, so it leads to a significant amount of thickening of the ice shell."

It won't happen like this on Earth, but this would be a bit like the Arctic ending up at the Equator, while everything beneath the crust stays in the same place.

Interestingly, if this polar wanderer hypothesis is true, then there should be a similar bulge on the opposite side. The researchers hope to confirm this in future.

Read more at National Geographic.

Top image: A natural color image of Ganymede acquired by NASA's Voyager 1 spacecraft.