The long, straight structures visible on the surface of Mars are not fault lines caused by the movement of tectonic plates, but are in fact a series of long, low ridges and hills called dykes. Geophysicists have argued for years that Mt Olympus on Mars - the largest volcano in the solar system - could only have been so big if there were no tectonic plates on the planet. Now numerical simulations by Dan McKenzie and Francis Nimmo from Cambridge University in the UK suggest that if the structures are indeed dykes, then they may have provided a heat source that have melted ground water. Evidence surrounding the dykes suggest they have caused enormous floods in the past (Nature 397 231).