The first large, stable mass of liquid water had been detected on Mars – boosting the chance of one day finding life on the planet.
Scientists using ground-penetrating radar found a 12-mile (about 20km) wide lake about a mile beneath the southern ice cap.
Salts in the lake are believed to have kept the water – which has a temperature as low as -68C (-90F) – from freezing over.
Liquid water is an essential requirement for life as we understand it.
Although the conditions in the lake may be too harsh for anything to survive, the fact it exists points to other potential liquid water on the Red Planet which may be warmer and less salty.
The exact depth of the water is not known and scientists say it could turn out to be a shallow, murky sludge.
Mars is now cold and dry, but 3.6 billion years ago was home to plenty of liquid water.
An Italian team made the discovery analysing a radar survey that was done between 2012 and 2015 by the Mars Express orbiter spacecraft.
One of the 29 samples showed unusually strong reflections.
Professor Roberto Orosei, from the University of Bologna, wrote in the journal Science: “Anomalously bright subsurface reflections are evident within a well-defined 20-kilometre-wide zone … which is surrounded by much less reflective areas.”
He said further analysis led them to “interpret this feature as a stable body of liquid water on Mars”.
The tool used to detect the water is called the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS).
It sends pulses that can penetrate the planet’s ice caps and give an indication of what is below the surface,
“It’s tempting to think that this is the first candidate place where life could persist [on Mars],” said Kirsten Siebach, a planetary geologist at Rice University in Houston.
Another expert, Alan Duffy, an associate professor at Swinburne University in Australia, said: “This is a stunning result that suggests water on Mars is not a temporary trickle like previous discoveries but a persistent body of water that provides the conditions for life for extended periods of time.”