The Sputnik Planum ice plain is the western lobe of a heart-shaped region close to the dwarf planet’s equator. In the center of Sputnik Planum, floating hills cluster in groups observed to be up to 12 miles across. The Challenger Colles, another cluster located further north on the plain, measures 37 miles by 22 miles.
Scientists believe that the hills are blocks of water ice—which is less dense than nitrogen-dominated ice—that float to the top of the plain.
The hills probably began as broken fragments of the rugged uplands surrounding the Sputnik Planum. The fragments were likely carried by nitrogen ice glaciers into the plain, where the convective motions of the glaciers pushed them into clusters.
In the above photo by NASA, the inset covers an area of 300 miles by 210 miles. It was taken on July 14, 2015, when the spacecraft was 9,950 miles from Pluto, mere minutes before the spacecraft the closest to the dwarf planet during its flyby.