The Mars robot, Spirit, has returned photographs of the Martian ground that seems to be evidence of a big volcanic explosion at the Home Plate area, a plateau of layered bedrock. Steve Squyres of Cornell University says "there is strong evidence that those layers are from a volcanic explosion. He says that the area near Home Plate is dominated by basaltic rocks. "When basalt erupts, it often does so as very fluid lava, rather than erupting explosively. One way for basaltic lava to cause an explosion is for it to come into contact with water---it's the pressure from the steam that causes it to go boom." This new finding gives more proof that water once flowed on Mars. The arrow in the picture points to a 4cm "bomb sag." Mars is a planet that has been considered to be geologically sterile for a long time.