Charon

Science Daily: Pluto’s ‘hulk-like’ moon Charon: A possible ancient ocean?

summary - Pluto's largest moon may have gotten too big for its own skin. Images from NASA's New Horizons mission suggest that Pluto's moon Charon once had a subsurface ocean that has long since frozen and expanded, pushing outward and causing the moon's surface to stretch and fracture on a massive scale.