The Mars that was and the Mars that is are two very different things. Three or so billion years ago, the Red Planet was awash with water, as now-dry riverbeds, deltas, and ocean basins reveal. But when the planet lost its magnetic field, it lost its protection from the solar wind, which stripped away much of the planet’s atmosphere and allowed most of its water to escape to space.

But the key here is “most.” There is plenty of water, in the form of ice, locked up in Mars’s polar caps—about the same amount as exists in Earth’s Greenland ice sheet. For exobiologists looking for possible life on Mars, water is essential—but only in its liquid state, so ice is kind of a dealbreaker. Now, however, a new study In Nature Astronomy suggests that in the south Martian pole at least, there may be a lake buried beneath the ice. It could measure as much as 30 km (18 mi.) across and is kept warm by geothermal heating, similar to the kind generated on Earth by radioactive isotopes or subsurface magma.