"This is the place on Mars where you have something that most resembles a habitat, a place where life could subsist", said planetary scientist Roberto Orosei of Istituto Nazionale di Astrofisica in Italy, who led the research published in the journal Science.
They obtained 29 sets of radar samplings, mapping out an area exhibiting a very sharp change in its associated radar signal, about 1.5 km below the surface of the ice and extending sideways about 20 km.
Here and on Mars, these lakes stay liquid because dissolved salts, and pressure from above, lower the water's freezing point.
Previously, there has been some suggestions about water on Mars, like droplets of water condensing on the Phoenix lander or as the possible cause of recurring slope lineae, which are seasonal dark streaks on Martian slopes.
Liquid water is considered as one of the necessary ingredients for life to emerge, and finding it on Mars is promising, but scientists have yet to find any evidence that the planet is or was habitable.
The huge body of water exists about one mile beneath the southern Martian ice cap and appears similar to Lake Vostok, which was found 2.4 miles beneath Antarctica and contained more than 3,500 species.
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If other reservoirs are detected and a network of glacial lakes is uncovered - like on Earth - then it could indicate that water has persisted on Mars for millions of years, said Orosei.
Researchers have long suspected evidence of water could be found on the Red Planet, a precursor for life on Mars.
"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 San Antonio, Texas, told AFP in an email. Further evaluation of the bright feature indicated an interface between the ice and a stable body of liquid water.
"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". Conditions most favorable to life generally involve having liquid water available for long periods of time, he said.
"The fact that it's buried underneath the surface isn't a big surprise, because liquid water can not exist on the surface of Mars, it's simply not possible because of the atmospheric pressure is too thin", Enright said.
Speaking in a recorded interview released by Science, Prof Orosei revealed that his team spent years checking their results before being confident enough to announce the discovery.
Indeed, scientists have found bacteria and other simple forms of life living in these kinds of extreme environments on our own planet, which use chemical reactions with salts and minerals to get the energy they need to live.