Underground caves found on Mars

Researchers from the US Geological Survey (USGS) have found underground caves on Mars. The seven sister cave holes have been named Dena, Chloë, Wendy, Annie, Abbey, Nikki and Jeanne. Glen Cushing from the USGS in Flagstaff, Arizona, got his first hint of the underground cave system from THEMIS (Mars Odyssey's Thermal Emission Imaging System) images of the Arsia Mons region near the equator of Mars. He spotted a system of pit craters, indicative of collapsed areas, and nestled among them half a dozen dark spots ranging in diameter from 100 to 252 metres.