South Polar Features (Natural Colors; credits: Lunar Explorer Italia)

The SHARAD (Shallow Subsurface Radar) looks for liquid or frozen water in the first few hundreds of feet (up to 1 kilometer) of Mars' Crust.
SHARAD probes the subsurface using radar waves within a 15- to 25-megahertz frequency band to get the desired, high-depth resolution.

The Radar Wave Return (RWR), which is captured by the SHARAD antenna, is sensitive to changes in the electrical reflection characteristics of rock, sand and any water that may be present in the Surface and Subsurface. Water, like high-density rock, is highly conducting, and has a very strong RWR. Changes in the reflection characteristics of the Subsurface, caused by layers deposited by geological processes in the ancient history of Mars, are also visible.