In July 2015, NASA's New Horizons spacecraft gave humanity its only close-up of Pluto. The spacecraft found that the dwarf planet, far from being a featureless and frigid orb, was an ostentatious world with epic impact craters, methane ice and nitrogen snow. Two of its mountains, Wright Mons and Piccard Mons, were suspected of being volcanoes. But instead of spewing molten rock, they would be built of, and capable of erupting, ice in a process known as cryovolcanism.