NASA announced that Pluto’s nitrogen ice glaciers appear to be ferrying some interesting passengers: isolated water ice mounds, which the agency believes may stem from the dwarf planet’s uplands.

These “hills” are located within Sputnik Planum, which is located in Pluto’s heart-shaped Tombaugh Regio. Measuring between one and several miles across, the hills are believed to be small versions of the mountains on Sputnik Planum’s western border. The recent announcement is based on images and data from NASA’s New Horizons mission, which completed a flyby of the dwarf planet in July 2015.

“Because water ice is less dense than nitrogen-dominated ice, scientists believe these water ice hills are floating in a sea of frozen nitrogen and move over time like icebergs in Earth’s Arctic Ocean,” according to NASA. “The hills are likely fragments of the rugged uplands that have broken away and are being carried by the nitrogen glaciers into Sputnik Planum. ‘Chains’ of the drifting hills are formed along the flow paths of the glaciers. When the hills enter the cellular terrain of central Sputnik Planum, they become subject to the convective motions of the nitrogen ice, and are pushed to the edges of the cells, where the hills cluster in groups reaching up to 12 miles across.”

At the top of the new image, which covers a region a little over 300 mi long and 210 mi wide, is a large area of mounds identified as Challenger Colles, which was named in honor of the crew lost in the Challenger explosion. According to NASA, the area measures 37 by 22 miles. Due to Challenger Colles close proximity to the “uplands,” the agency believes it may be beached in an area where the nitrogen ice is particularly shallow.

The new image was taken when New Horizons was about 9,950 mi from Pluto.