The most recent finding from New Horizons show that ice bergs have broken off from the hills surrounding the Sputnik Planum, a glacier of nitrogen ice, and are floating slowly across its surface, eventually to cluster together in places like the Challenger Colles, informally named after the crew of the space shuttle Challenger, which was lost just over 30 years ago. The feature is an especially high concentration of icebergs, measuring 37 by 22 miles. The icebergs float on the nitrogen ice plain because water ice is less dense than nitrogen ice.

Source: NASA announces that Pluto has icebergs floating on glaciers of nitrogen ice