The news is quite exciting for the prospects of life on the Red Planet, because water is essential for life.
So first, you must find water.
Artist's impression of the Mars Express spacecraft probing the southern hemisphere of Mars. But the radar reflections could be produced by a layer of water less than a meter thick, so it could be just a thin layer of liquid lining the base of the ice sheet. The depth of the water is not known.
An Italian team of scientists detected the lake while carrying out a radar survey using the Mars Express spacecraft.
One of the 29 samples showed unusually strong reflections.
The continuous white line at the top of the radar results above marks the beginning of the South Polar Layered Deposit; a filo pastry-like accumulation of water ice and dust.
"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. Its ground-penetrating radar detects boundaries between structures of different dielectric permittivity - a measure of a material's electrical polarisation under the influence of an external electric field.
Detection via radar signals means that the thickness "would need to be at least several tens of centimeters thick", according to a statement by the European Space Agency, which described the body of liquid water as "possibly laden with salty, saturated sediments".
Mars Express orbiting Mars with a cross section of radar echoes superimposed and tilted 90 degrees.
The discovery could offer fresh clues about how Earth's neighbor so profoundly transformed billions of years ago from a warmer, wetter world to its freeze-dried state today, according to Scientific American.
Either way, the discovery greatly increases the chances of extraterrestrial life existing on Mars.
"This is a discovery of extraordinary significance, and is bound to heighten speculation about the presence of living organisms on the Red Planet", said Fred Watson of the Australian Astronomical Observatory.
The high levels of salt in the water might make the environment tough but not impossible for life.
The discovery is the latest of many breakthroughs by Nasa's Mars missions.
"Our mantra back then was 'follow the water.' That was the one phrase that captured everything", Hubbard said.
The radar data showed that the lake contained a large amount of salt, Prof Orosei added.
To know for certain if this lake actually exists is not going to be easy, Orosei says. The chambers allow for experiments in pressure and temperature environments similar to conditions found on other planets.
Still, the more measurements scientists can collect, the more confident they can be.