NASA says its Mars Odyssey spacecraft has spotted seven possible cave entrances on Mars. These dark features, several hundred feet across, have a more constant temperature from day to night than the ground surrounding them (as shown in the infrared images at center and right). If the “Seven Sisters” holes turn out to be caves, it’s possible they could provide a protected niche for past or present life on Mars—or serve as underground shelters for future human colonization.—Dawn Stover

Image: NASA/JPL-Caltech/ASU/USGS