Using a radar instrument called Marsis on the Mars Express orbiter, researchers from the European Space Agency beamed radar waves at the Martian surface. Radar is able to penetrate the ground. In fact, similar technology has been used here on Earth to find ancient and dry riverbeds under the Sahara and remains of ancient civilizations that are invisible to explorers who walk over the ground.
As the satellite passed near the Martian south pole, they saw reflections from deep underneath the planet's surface that indicated the presence of liquid water. This water isn't easily accessible; it is located about a mile (1.5 kilometers) below the surface.
It is unclear just how much water is involved. Initial indications are that it is a body about 12 miles (20 kilometers) wide. It is not possible to know how deep it is, but it is at least three feet (one meter) thick. If these dimensions are right, this is a considerable amount of water.
The temperature of this water is also important. It's extremely cold -- -90 degrees Fahrenheit (-70 Celsius). Normally water at this temperature would be frozen ice, but the Martian surface has a plentiful reservoir of magnesium, calcium and sodium salts called perchlorates, which can keep water liquid at very low temperatures. Thus, this particular lake would be saltier than the Earth's oceans. However, this isn't an insurmountable problem for future explorers. If they can get to this water that is a mile underground, desalinization techniques exist.
The existence of water on Mars isn't an enormous surprise. We have known for a long time that the surface of Mars was once much warmer and wetter. Chemical tests by earlier probes and images of complex dry river beds attest to that. Further, the existence of ice below the planet's surface has been reported by surveys using other ground penetrating radar.
