This approximate true-color rendering from NASA's Mars Exploration Rover Spirit shows a set of darker rocks dubbed "Toltecs" lying southeast of the rover's current position. These rocks are believed to be basaltic, or volcanic, in composition, because their spectral properties match those of other basaltic rocks studied in Gusev Crater. Scientists hope to use these presumably unaltered rocks as a geologic standard for comparison to altered rocks in the area, such as "Clovis." This image was taken with the panoramic camera's 600-, 530-, and 480-nanometer filters on sol 220 (Aug. 15, 2004).