The only dwarf planet located in the inner solar system—that is, the area before you get to Jupiter—Ceres is the largest object in the asteroid belt between Mars and Jupiter. Still, it's dwarfed (see what we did there?) by its dwarf-planet pal Pluto, which is 14 times more massive. In 2005, astronomers estimated that as much as 30 percent of its total mass could be made up of water ice. In 2016, data from NASA's Dawn spacecraft all but confirmed that estimate: at high latitudes, water ice may make up 27 percent of its mass, although the concentration drops near the equator.
That's huge. Take Vesta, the second biggest object in that asteroid belt. Ceres's hydrogen content—the telltale sign of water—is more than 100 times that of Vesta, and in a more even distribution. What's more, another study even found ice on the surface itself.