This image looks remarkably like groves of trees growing among Martian dunes. But, the trees are an optical illusion. They are actually dark streaks of sediment on the downwind side of the dunes. They were created by escaping gas from the evaporating carbon dioxide ice below. The bottom of the ice melts into vapor and moves toward holes in the ice, carrying dark sediment along with it that is then deposited when the gas escapes.

Wired Science: Strange Places on Mars: What Do You Want to See Next?