A probable lake of liquid water brine has been identified beneath the south polar ice cap on Mars. The lake sits about a mile under the ice, is about 12 miles across (20 km) and at least 3 feet deep. Salts in the martian ice are likely to be responsible for keeping the water liquid, and this may be one of many such lakes yet to be found on the Red Planet.
The discovery was made by a science team from the European Space Agency (ESA) led by planetary scientist Roberto Orosei of the National Institute of Astrophysics in Bologna, Italy. They found the lake by analysing data acquired by the MARSIS instrument on board the orbiting spacecraft Mars Express, which uses radar waves to probe beneath its surface. Similar lakes have been found hidden below the ice in Greenland and Antarctica using the same process.
The prospect of finding liquid water lakes beneath the surface of Mars is not a new idea (see the book: "Secret Mars" 2nd ed. page 94), but this is by far the largest ever discovery of liquid water on Mars and significantly increases the prospect of finding life – although drilling through a mile of solid ice to reach it will prove a challenge.