Radio waves beamed down to the surface by Marsis penetrated through the ice and bounced back to the spacecraft.
A total of 29 sets of radar samplings showed a "very sharp change in its associated radar signal", allowing scientists to map the outlines of the lake.
Given its location beneath the polar ice cap, the water is expected to be below the freezing point of water.
The team that wrote the study, including lead author Professor Roberto Orosei, have ruled out any other causes for the brightness. 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.
There have been previous signs of liquid and water activity on Martian slopes, but never before a "stable body" - which strengthens the notion of alien life inhabiting the planet. In comparison, salty ocean water freezes at 28.4 degrees Fahrenheit, according to the National Oceanic and Atmospheric Administration.
Either way, the discovery greatly increases the chances of extraterrestrial life existing on Mars.
While it's always been known that the planet has substantial ice deposits, this is the first time scientists have detected a stable body of water in liquid form on the planet.More news: Trump 'Very Happy' with North Korea Nuclear Weapons Talks
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Almost 4.5 billion years ago, Mars had six and a half times as much water as it does now and a thicker atmosphere. Primitive life may have evolved on the planet then, only to be snuffed out when the world lost most of its atmosphere and became an arid frozen desert. "Those are not ideal conditions for life to form", Siebach said.
Researchers said the lake looked similar to the ones found beneath the Antartic and Greenland.
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.
Chevrier said he also wanted to know how old the body of water might be.
"Maybe this could even be the trigger for an ambitious new Mars mission to drill into this buried water-pocket - like has been done for sub-glacial lakes in Antarctica on Earth", he added. "A lake, not some kind of meltwater filling some space between rock and ice, as happens in certain glaciers on Earth", Orosei told the British media giant.
"This is certainly not a very pleasant environment for life", he said.
However, Stillman, who was not involved in the research, said another spacecraft, or other instruments, need to be able to confirm the discovery.
"But there are terrestrial organisms that can survive and thrive, in fact, in similar environments".