Liquid water is vital for biology, so the finding will be of interest to researchers studying the potential for life elsewhere in the Solar System.
But the lakes are also thought to be extremely salty, which could make it difficult for any microbial life to survive in them.
Mars' thin atmosphere means that the presence of liquid water on the surface is a near-impossibility. But water could remain liquid below ground.
The lakes are spread over about 75,000 square kilometres — an area roughly one-fifth the size of Germany. The largest, central lake measures 30 kilometres across, and is surrounded by 3 smaller lakes, each a few kilometres wide.
The researchers expect the water must be incredibly salty to stay liquid at low temperatures. A separate 2019 study suggested volcanic activity might help keep the water from freezing, but the current paper leans heavily into the salt concept.
Source: S.E. Lauro et al. Multiple subglacial water bodies below the south pole of Mars unveiled by new MARSIS data. Nature Astronomy. Published online September 28, 2020. doi: 10.1038/s41550-020-1200-6.