Astronomers suspect Tombaugh Regio, nicknamed the Heart of Pluto, is the result of a collision with a large object that sunk into the dwarf planet rather that generating an impact crater. That suggests Pluto has subsurface seas which is supported by the Kiladze cryovolcano. The belt of chasms around Pluto's moon Charon suggests it too once had a subsurface ocean that froze, expanding to produce the surface cracking.
—David Foss