Some deposits have been mapped by the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS), an instrument on the European Space Agency's Mars Express orbiter, which launched in 2003. The team ruled out that carbon dioxide ice or other features created the spot and were left with one explanation-a lake of liquid water.
Between May 2012 and December 2015, MARSIS was used to survey the Planum Australe region, which is in the southern ice cap of Mars. When pointed at the surface ice caps of the planet, it measures how radio waves penetrate and reflect back to the spacecraft.
Over the last ten years, different Mars missions have found increasingly more evidence of water on Mars, including large sheets of ice under the surface and salty mud flows on the surface. This kind of lake stays liquid because the water mixes with salts like magnesium, calcium, and sodium to form a brine which lowers the water's freezing point. The Mars Express team (otherwise known as MARSIS) believe that this new area they have found is actually a lake of sorts sitting below the Martian surface.
"This took us long years of data analysis and struggles to find a good method to be sure that what we were observing was unambiguously liquid water", said study co-author Enrico Flamini, chief scientist at the Italian Space Agency during the research.
Mars long ago was warmer and wetter, possessing significant bodies of water, as evidenced by dry lake beds and river valleys on its surface.
Unlike many Mars discoveries, this particular revelation is all but definitive.
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The body of water appears similar to underground lakes found on Earth in Greenland and Antarctica.
But researchers are excited about the potential for future finds, because if liquid water could be found at Mars' south pole, it might be elsewhere too.
Such radars are useful when searching for liquid water, "because water is a very strong radar reflector", he says.
But because of the thin atmosphere and the cooling of the planet, most of its water is locked up in ice.
The body of water is about 20 kilometres across and, if confirmed, would be the first evidence of permanent water on the Red Planet. Although this is just one detection, the team wrote, "there is no reason to conclude that the presence of subsurface water on Mars is limited to a single location".
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"All the technology to drill through this ice to the lake doesn't exist yet so it will probably take at least another 25 years before we will be examining this".
"This finding is potentially of enormous significance", says Clifford, who was not involved with the study. "I see cells in it, but it's so briny that the cells-I can't get them to metabolize, " he said, referring to the chemical processes that all organisms undertake to survive. The nature of reflections off the subterranean features offer scientists information about exactly what lies beneath the surface and the data says there is liquid water. But Clifford holds out hope subsurface geothermal hotspots like those that power volcanoes and hot springs on Earth could sufficiently heat portions of the Martian underworld to allow liquid reservoirs to exist there without the need for life-sabotaging salt levels. Water is now driving NASA's exploration into the outer solar system, where ocean worlds - like Jupiter's moon Europa and Saturn's moon Enceladus - hold the potential to support life.