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. ESA's Mars Express spacecraft obtained these images using the HRSC during orbit 902 on 2 October 2004 with a ground resolution of approximately 14.3 metres per pixel. The images were acquired in the region of Pavonis Mons, at approximately 0.6° South and 246.4° East. Pavonis Mons, rising roughly 12 km above the surrounding plains, is the central volcano of the three 'shield' volcanoes that comprise Tharsis Montes. Gently sloping shield volcanoes are shaped like a flattened dome and are built almost exclusively of lava flows. The dramatic features visible in the colour image are located on the south western flank of the volcano. 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. Similar tubes are well known on Earth and the Moon. Pit chains, strings of circular depressions thought to form as the result of collapse of the surface, are also visible within the colour image.