Judging from how radar signals beamed down to Mars bounced back to the spacecraft, the scientists believe they're seeing a 20-kilometer wide shallow body of water, capped by 1.5 kilometers of hard ice.
Between May 2012 and December 2015, Roberto Orosei with International Astronomical Union and his Italian colleagues used MARSIS to survey a region called Planum Australe, located in the southern ice cap of Mars. So while a layer of ice and a layer of rock may both be weakly reflective, the boundary that occurs when ice sits on top of rocks will create a strong reflection that can be detected by an orbiting radar.
A huge lake of salty water appears to be buried deep in Mars, a new study says.
Salts in the lake are believed to have kept the water - which has a temperature as low as -68C (-90F) - from freezing over.
The reservoir - which is approximately 20 kms in diameter and is shaped like a rounded triangle - is the first stable body of liquid water ever found on Mars.
The location's radar profile resembled that of subglacial lakes found beneath Earth's Antarctic and Greenland ice sheets. However, the presence of liquid water at the base of Martian polar caps was first hypothesized in a study 31 years ago.
Australian experts have applauded the find, saying the discovery of liquid and frozen water resources is key due to their ability to produce fuel and provide life support for astronauts during deep space missions, allowing lower cost launches and space operations.
Dmitri Titov, ESA's Mars Express project scientist, added: "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 neighbour planet and its habitability".
Lake beds like those explored by Nasa's Curiosity rover show water was present on the surface of Mars in the past.
Scientists from the Italian space agency have discovered a large underground lake on Mars, which could significantly increase the probability of life on the planet. Further evaluation of the bright feature indicated an interface between the ice and a stable body of liquid water.
He said: "Magnesium, calcium, and sodium could be dissolved in the water to form a brine".
Last week data were released revealing that there are organic molecules on Mars.
Associate Professor Alan Duffy, lead scientist of Australia's science Channel, said the ending of Total Recall where Arnold Schwarzenegger melted vast ice reserves just became less science fiction and more science fact.
The researchers noted that together, with the pressure of the, this lowers the melting point, allowing the lake to remain liquid, as happens on Earth.