In recent years, NASA has launched wave after wave of robots to Mars. One of their goals is to find evidence that liquid water once flowed on the surface of the Red Planet and carved its spectacular, Grand Canyon-like terrain. But is the water hypothesis all wet? An iconoclastic Australian geoscientist claims the fourth planet from the sun is as dry as a bone – and always has been. American researchers initially scoffed at Nick Hoffman’s thesis. But now they’re starting to take him more seriously. There’s more at stake than the popular theory that ancient rivers, lakes and perhaps oceans carved the rust-red Martian terrain. According to Hoffman, those mountains, mesas and canyons were dug by epic floods of liquefied carbon dioxide gas, not by water.