Red Planet Radar: Probing the Martian Underground for Water

Using a radar system through European Space Agency (ESA), scientists are planning to search for signs of water 3 miles (5 kilometers) below the surface using radio waves. Aboard the Mars Express, there is a "Mars Advanced Radar for Subsurface and Ionospheric Sounding instrument, called MARSIS" that will be responsible for searching the planet. The searching will be accomplished while the Mars Express is in orbit using a 131 foot antenna with a 6 foot wide point. MARSIS will then send two low radio frequencies to produce a radar surrounding. For about two years (687 days) MARSIS will be able to observe Mars, and allow scientists to develop three dimensional images of Mars' crest.
http://www.space.com/businesstechnology/technology/marsis_radar_030430.html