With its heavily cratered surface and no signs of activity, Saturn's moon Mimas is the last place you'd expect to find a global ocean beneath its surface. Based on a detailed analysis of its orbital motion, a team of researchers from Paris Observatory and CNRS have demonstrated that, contrary to all expectations, Mimas has an ocean at a depth of just 20-30 km. It is thought to have formed less than 20 million years ago. This discovery makes Mimas a unique target for studying the primitive conditions in which life first appeared.