English: This image, taken by the High Resolution Stereo Camera (HRSC) on board ESA's Mars Express, shows Pavonis Mons, the central volcano of the three 'shield' volcanoes that comprise Tharsis Montes. Pavonis Mons, rising roughly 12 km above the surrounding plains, is the central volcano of the three 'shield' volcanoes that comprise Tharsis Montes. The dramatic features visible in the image are located on the south western flank of Pavonis Mons. Researchers believe these are lava tubes, channels originally formed by hot, flowing lava that forms a crust as the surface cools. Lava continues to flow beneath this hardened surface, but when the lava production ends and the tunnels empty, the surface collapses, forming elongated depressions. The long, continuous lava tube in the northwest of the image extends over 59 km and ranges from approximately 1.9 km to less than 280 m wide.