A group of 7th graders in California were examining orbiter photos Martian lava tubes as part of a project in the Mars Student Imaging Program run by NASA and Arizona State University when they found a pit that appears to be a skylight into a cave. [right, pit is in the photo center. Photo credit: NASA/JPL-Caltech/ASU]
An article on ScienceDaily.com says the cave is on the slope of an equatorial volcano named Pavonis Mons.