http://www.newsweek.com/ghost-dunes-discovered-mars-and-they-could-be-hiding-ancient-microbes-1020130

Researchers have discovered around 800 crescent-shaped pits on Mars. They are the imprints of long-vanished ancient sand dunes that once rose high above the Red Planet’s surface, according to a study published in the American Geophysical Union's (AGU) Journal of Geophysical Research.

The pits were formed around two billion years ago. Back then Mars would have looked very different to how it appears today, with flowing water on its surface and active volcanoes.