Never in history has a robot behaved so much like a human geologist.

NASA's Opportunity rover provided many lines of evidence suggesting its landing site was once drenched in water. While some of the evidence involved complex observations and analysis of chemicals and minerals, the discovery was made possible by the robot's ability to survey its surroundings and then drill into rocks -- just like a field geologist.

Opportunity sees its target with human-like, 3-D vision. Early in the mission, it took in a 360-degree color panoramic view of its horizon from the height of human eyes. Closer inspection of a curb-height rock ledge showed the rocks were built in layers, which scientists ultimately determined was the result of sediments being laid down over time.

The rover's rock abrasion tool (RAT) worked as planned, drilling into various rocks in the exposed ledge in the shallow depression where Opportunity landed. It was the next best thing to cracking a rock open with a hammer. Among the most compelling signs of past water on Meridiani Planum -- made possible by the RAT and other high-tech analysis tools -- were high concentrations of salt left behind after water altered the chemical composition of the rocks.

Scientists presented all the evidence at a March 2 press conference. There promises to be much more to learn from Opportunity and its twin rover, Spirit. Meanwhile, see how scientists pictured water on Mars in a new image gallery of discovery.

• Mars Rovers: Complete Coverage