Reviewing data from the New Horizons probe's flyby of Pluto and its moons, NASA scientists believe the dwarf planet's largest satellite once had a huge subsurface ocean.

In a news release published Thursday, NASA detailed how a giant canyon on Charon's surface may actually be the ocean, albeit frozen over long ago. The ocean then apparently expanded so much it fractured the moon's surface.

"The outer layer of Charon is primarily water ice. This layer was kept warm when Charon was young by heat provided by the decay of radioactive elements, as well as Charon's own internal heat of formation," read the release. "Scientists say Charon could have been warm enough to cause the water ice to melt deep down, creating a subsurface ocean. But as Charon cooled over time, this ocean would have frozen and expanded (as happens when water freezes), lifting the outermost layers of the moon and producing the massive chasms we see today. "

Gizmodo reported in October the canyon where the new analysis placed the subsurface ocean is four times as long as the Earth's Grand Canyon and twice as deep. At the time, NASA scientists were fascinated with the canyon, as it appeared to be a massive fracture in the moon's surface.

Charon is the largest of Pluto's five known moons. The New Horizons probe captured some stunning images of it last summer in addition to its main objective of photographing Pluto.