It would be at least 20 kilometers wide and would have to be "several tens of centimeters thick" - otherwise the radar could not have detected it.
MARSIS surveyed Mars' Planum Australe region between May 2012 and December 2015 and utilized radar pulses, sending them through the surface and the polar ice caps, ultimately measuring how the radio waves came back. Subsurface echo power is color coded and deep blue corresponds to the strongest reflections, which are interpreted as being caused by the presence of water.
The radar profile of this area is similar to that of lakes of liquid water found beneath the Antarctic and Greenland ice sheets on Earth, suggesting that there might be a subglacial lake at this location on Mars, according to the study.
Mars is very cold, but the water might have been kept from freezing by dissolved salts.
Scientists will continue to work at discovering more water in order to figure out whether the newfound lake is a one-off, or in fact one in a network of resevoirs.
Our quest on Mars has been to 'follow the water, ' in our search for life in the universe, and now we have convincing science that validates what we've long suspected.
A few years ago, biologists found more than 3,500 unique gene sequences in Lake Vostok which had been isolated for more than 15 million years; Lake Vostok gets no sunlight with it being 4,000 metres below the ice and has a recorded temperature of -89.2c, showing life to be hardy.
The discovery, which has major implications for the chances of life surviving on the Red Planet, was made by an orbiting European probe using ground-penetrating radar.
"This water would be extremely cold, right at the point where it's about to freeze".
The tool is called the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), and was created to find subsurface water by sending radar pulses that penetrate the surface and ice caps.
"This subsurface anomaly on Mars has radar properties matching water or water-rich sediments", said Roberto Orosei, lead author of the paper and principal investigator of the MARSIS experiment.
Boffins have discovered the first-ever liquid water lake on Mars and is thought to be the largest body of liquid water ever found on the Red Planet. "Those are not ideal conditions for life to form", Siebach said.
After decades of trying to answer the question of whether Mars has liquid water, scientists appear to have found the answer, and the implications of their discovery could be huge. Learning more about these caps can reveal Mars' climate history.
"Many scientists have said "where there is water, there is the potential for life".