With the recent Pluto photoshoot by NASA's New Horizons spacecraft, a lot of fantastic new mysteries have come to light.
One of these are the so-called 'Floating Hills' found in the Sputnik Planum terrain. Since water ice is less dense than nitrogen ice it is believed that these ice hills are giant icebergs 'floating' in an ocean of frozen nitrogen.
Their origin is suggested to be from the glaciers. Like on earth, icebergs have broken off the main glaciers and drifted out into the nitrogen sea where they now float and cluster into groups reaching up to 12 miles across. One particular cluster in the Challenger Colles seems to measure 37 to 22 miles.