Evidence is mounting for an ocean—with semi-frozen water—at Pluto, buried beneath its frozen heart.

These latest findings are based on observations by NASA’s New Horizons interplanetary space probe, which made an unprecedented flyby of Pluto last year, according to Associated Press.

Scientists said that Pluto may have rolled over on its axis eons ago, the result of tidal forces with its jumbo moon Charon. The extra weight of an underground sea is the most likely explanation, they said.