As we reported recently, NASA’s flyby of Pluto with the New Horizons spacecraft has yielded yet another fascinating discovery, this time of Pluto’s moon Charon — an amazing cosmic body that holds mysteries that continue to make it as interesting as its host planet.

The latest discovery on Charon is that it once had a large subsurface ocean, which eventually froze over and burst through the crust of the moon, resulting in huge cracks on the surface, according to a Washington Post report. Charon is covered in water ice and scientists think that at one point it could have been warm enough to be in liquid form, pooling deep below the surface until the heat dissipated and froze the subsurface ocean.

But there are many other fascinating aspects about this moon, which is massive compared to the size of its host planet at half the size. Scientists first became wowed by Charon’s mysterious surface as New Horizons drew near in the middle of last year. They noticed a giant, dark-red blob at the north pole, causing them to nickname it “Mordor” referring to the evil land in “Lord of the Rings,” according to a TechInsider report.

But what made this strange appearance? It’s possible that the north pole of Charon is made of a different material than the rest of the moon, and the moon may have in fact stolen some of the gases that Pluto has been losing from its atmosphere, which then freeze into a solid when they reach Charon’s north pole, never reaching the liquid state as Charon is incredibly cold. Then, Charon rotates in a way so that the sun can hit the frozen gases with solar radiation, which may turn it into materials called “tholins.”

Tholins have the texture of tar and are different chemically than how they started. They also tend to look a dark red when dry. So that’s what it may be: tholins built up over millions of years.

The gases would include nitrogen, carbon monoxide, and methane.

Scientists also observed a strange surface feature that continues to baffle them. A photo from New Horizons appeared to show a mountain rising from a depression — a “mountain in a moat,” as they referred to it. Geologists still aren’t sure how it got there.