This animation shows the Mars Exploration Rover Spirit digging a trench in the fine-grained soil lining the bottom of the shallow depression dubbed "Laguna Hollow."

The trench is 3 inches (7 centimeters) deep and was created after the rover dragged one of its wheels back and forth across the surface 11 times. The images above were taken by Spirit's front hazard-avoidance camera at intervals during the digging after the wheel had returned to a starting position.

The whole operation lasted about two hours. A similar trench was dug on the other side of Mars by Spirit's twin, Opportunity, is 4 inches (10 centimeters) deep and took only six sweeps to complete, indicating that the soil there is more loosely packed.

Spirit excavated its trench by repeating two basic steps: first, it spun its left front wheel in reverse to dig; second, it turned left and spun the same wheel in the forward direction. To widen the trench, each pass back and forth began a little farther to the right. By the end, the rover had swiveled 40 degrees.

Credit: NASA/JPL