NASA’s New Horizons probe continues to transmit new information about Pluto and its moons after its historic flyby last summer, and the latest findings have added to the list of mysteries surrounding the worlds on the outer edge of the solar system. According to a report from Phys.org, new images suggest that one of Pluto’s moons, Charon, used to have a massive subsurface ocean.

It’s not very likely that the ocean is still there – scientists say it froze and expanded long ago, creating a network of massive fractures covering Charon’s surface.

Photos of Charon from the New Horizons probe taken last July revealed a series of tectonic faults that created ridges, scarps, and valleys as much as four miles deep. The fractures were caused by the subsurface ocean tearing through the moon’s crust.

The surface of Charon is mainly made up of water ice. In the moon’s early days, internal heat and high levels of radiation likely meant that much of this layer was liquid. Scientists believe the subsurface ocean formed when water on Charon’s surface leaked into the moon’s inner layers.

The image snapped by the New Horizons probe shows a feature on the moon’s surface called Serenity Chasma, a massive ravine near the equator. The series of chasms is one of the largest examples ever seen in the universe, and helps visualize the scale of the subsurface ocean as it froze and expanded. The feature runs for at least 1,100 miles, compared to the Grand Canyon’s 277.

The Long-Range Reconnaissance Imager, also known as LORRI, was responsible for snapping the image as New Horizons flew by in July.

A press release from NASA describing the details of the discovery can be found here.