Dr. Steven Squyres is the principal scientific investigator for NASA's Mars Exploration Rover Project. Twin robot geologists, Rovers Spirit and Opportunity, have both found evidence for water-related processes at their landing sites. In Gusev crater, Spirit found only trace evidence for the action of water on the lava-covered plains of the crater floor. The older rocks in the Columbia Hills, however, show evidence for substantial alteration by water. At Meridiani Planum, Opportunity has found layered sedimentary rocks precipitated from liquid water. After deposition, the rocks underwent a complex history of modification by water including periodic inundation by shallow surface water, evaporation, and drying. Environmental conditions at Meridiani may have been suitable for supporting life at some point in the planet's history.