Cool beans! Using Radar to find Mars Aquifers.
Red Planet Radar: Probing the Martian Underground for Water
From the article;
European researchers hope to pin down exactly where Mars is hiding its water by looking through the planet rather than on its surface.
A ground penetrating radar system aboard Mars Express, a European Space Agency (ESA) probe set to launch in early June, will use radio waves to map out any water reservoirs as deep as 3 miles (5 kilometers) below the surface.