Scientists are eager to find signs of contemporary water, because such discoveries are key to unlocking the mystery of whether life ever formed on Mars in its ancient past, or if it might persist today.
The temperature of the water in the 12-mile-wide lake is believed to be minus-90 degrees Fahrenheit.
Liquid water is an essential requirement for life as we understand it.
While Pettinelli said SHARAD's radar uses too high a radar frequency to see the signal, another researcher had a another take.
Artistic impression of the Mars Express spacecraft probing the southern hemisphere of Mars, superimposed to a color mosaic of a portion of Planum Australe. Now, researchers claim that they've discovered what might very well be a persistent Martian "lake" sitting just below the surface.
MARSIS is a low-frequency radar and altimeter that features operation altitudes up to 800 km above the Martian surface for subsurface sounding and up to 1200 km for ionospheric sounding.
"This is the first body of water it has detected, so it is very exciting", David Stillman, a senior research scientist in the Department of Space Studies at Southwest Research Institute in Texas, told AFP in an email. In Earth's polar regions, the pressure of the overlying ice lowers its melting point, and geothermal heat warms it from below to create the subglacial lakes.
Experts suspect there has been water on Mars for around 4bn years - and it might contain hints of life from that time.
"This thrilling discovery is a highlight for planetary science and will contribute to our understanding of the evolution of Mars, the history of water on our neighbor planet and its habitability", Titov said.
Whether microbial forms of life could lie within is a matter of debate. Traces of water have been found in its atmosphere and leeching through its soils, but efforts to locate bodies of water - a likely place to find life as we know it - have not yet been successful.
The lake resembles one of the interconnected pools that sit under several kilometers of ice in Greenland and Antarctica, says Martin Siegert, a geophysicist at Imperial College London, who heads a consortium trying to drill into Lake Ellsworth under West Antarctica. Orbiters have also revealed vast glaciers residing just under the surface, potentially accessible to any future explorers or even colonists that go there in the future.
The reason for the reflections at these boundaries has to do with the electrical properties of the materials, and according to Stuurman, liquid water has radically different electrical properties than rock or ice.
Bramson said the Mars radar community remained curious about why SHARAD didn't detect water below the south pole.