Three new underground lakes have been detected near the south pole of Mars.
Scientists also confirmed the existence of a fourth lake - the presence of which was hinted at in 2018.
Liquid water is vital for biology, so the finding will be of interest to researchers studying the potential for life elsewhere in the Solar System.
But the lakes are also thought to be extremely salty, which could pose challenges to the survival of any microbial life forms.
Billions of years ago, water flowed in rivers and pooled in lakes on the Martian surface. But Mars has since lost much of its atmosphere, which means water can't stay liquid for long on the surface today.
However, it's a different matter underground.
Referring to the sub-surface lakes, co-author Dr Roberto Orosei, from Italy's National Institute of Astrophysics in Bologna, told BBC News: "It's even more likely that these bodies of water existed in the past.
"Of course, the implication of this is that you would have a habitat or something that resembles a habitat... that lasted throughout the history of the planet,"
"As Mars was undergoing its climatic catastrophe and turning from a relatively warm planet - though it's not clear how warm - to a frozen waste, there was a place where life could adapt and survive."
The latest discovery was made using data from a radar instrument on the European Space Agency's (Esa) Mars Express spacecraft, which has been orbiting the Red Planet since December 2003.
Is there any rough figure on when the last time was that surface water existed?
For an interesting theory on how most of Mars' atmosphere was lost, read "Worlds in Collision" by Emanuel Velikovsky, published in 1950.
I am looking forward to finding fish on Mars.