https://phys.org/news/2018-07-huge-liquid-lake-beneath-southern.html

One of the suite of instruments carried by Mars Express is MARSIS (the Mars Advanced Radar for Subsurface and Ionosphere Sounding), which allows researchers to use radar to study features beneath the planet's surface.

Using observations spanning a period of four years, a team of researchers from Italy found evidence of a large lake of salty water, buried 1.5 kilometres beneath Mars' southern polar cap. That lake is at least 20 kilometres across, and seems to be a permanent feature.