The Mars Reconnaissance Orbiter and its SHARAD (Shallow Subsurface Radar) instrument, which will seek liquid or frozen water in the first few hundreds of feet (up to 1 kilometer) of Mars' crust. SHARAD will probe the subsurface using radar waves using a 15-25 megahertz frequency band in order to get the desired high-depth resolution.

In this image the SHARAD instrument has detected subsurface water which I've shown in a cutaway view. The SHARAD antennae are the two white poles sticking out of MRO's back. The radar pulses emanating from the antennae have been simplified. This image only shows the downward pulses but in reality there would be upward pulses as well. The SHARAD instrument is a joint project of both NASA and ASI, the Italian Space Agency.