A “Lake Superior”-sized body of underground ice, and that “represents less than one percent of all known water ice on Mars”! It’s protected from evaporation (actually sublimation) by a layer of soil. The ice is not pure water, but is mixed with a fair bit of soil/dust/etc
Originally shared by NASA Jet Propulsion Laboratory
Water ice makes up half or more of an underground layer in a large region of Mars halfway from the equator to the north pole.