This stereo view combines a pair of images taken by the microscopic imager on NASA's Mars Exploration Rover Spirit during the 1,925th Martian day (sol) of Spirit's mission on Mars (June 2, 2009). Rover team members used the microscopic imager on the rover's arm to accomplish something never intended during the design of the rover or that camera -- getting a look underneath the rover. They did so to get a better understanding of Spirit's predicament, with wheels embedded deeply enough in soft soil at a site called "Troy" for the rover to be at risk of getting hung up on a rock under the rover. The dark triangular shape is a rock that is either touching or nearly touching Spirit's underbelly.
The two images combined here come from two camera positions, one slightly to left of the other, yielding a three-dimensional view when seen through red and blue glasses with the red lens on the left. The microscopic imager took one of the pair during Sol (Martian day) 1925 of Spirit's mission on Mars (June 2, 2009) and the other during Sol 1990 (Aug. 8, 2009). The team had not commanded any driving moves by Spirit in the interim while it was running experiments with a test rover on Earth to evaluate possible maneuvers for getting Spirit away from Troy.
The microscopic imager is designed to focus on rock or soil targets 6 centimeters (2.4 inches) away. It rides on the end of the rover's robotic arm so that it can be placed close to targets for inspection. It cannot focus on objects as far away as the rover underbelly and rocks that are visible in this image despite being out of focus.
The team has used this image as an aid in planning a drive strategy for Spirit. The rock underneath presents a risk of high-centering the rover if the wheels sink much further into the soil.