Whatever the outcome, the researchers say it would take years to verify whether something is living in the reservoir.
Italian researchers said evidence gathered from the radar signals of an orbiting spacecraft suggests a 12-mile-wide reservoir buried near the planet's south pole, The Associated Press reported.
"This is just one small study area; it is an exciting prospect to think there could be more of these underground pockets of water elsewhere, yet to be discovered", Orosei said in a statement on the European Space Agency's website. It may not necessarily be warmer down there, but the pressure of the glacier above, plus the possibility of salt in the water, would lower the water's melting point.
Previously, there has been some suggestions about water on Mars, like droplets of water condensing on the Phoenix lander or as the possible cause of recurring slope lineae, which are seasonal dark streaks on Martian slopes.
Almost 4.5 billion years ago, Mars had six and a half times as much water as it does now and a thicker atmosphere.
The scientists expect the water is very salty, otherwise it would freeze.
"The fact that it's buried underneath the surface isn't a big surprise, because liquid water can not exist on the surface of Mars, it's simply not possible because of the atmospheric pressure is too thin", Enright said. They would have to drill through the ice first to sample the water below.
"This took us long years of data analysis and struggles to find a good method to be sure that what we were observing was unambiguously liquid water", said study co-author Enrico Flamini, chief scientist at the Italian Space Agency.
"Nobody dares to propose that there could be any more complex life form", Orosei said.
It is the largest body of liquid water ever found on the Red Planet. But there hasn't been evidence of stable bodies of water until now, the researchers said.
However, Stillman, who was not involved in the research, said another spacecraft, or other instruments, need to be able to confirm the discovery.