This is Europa, moon of Saturn, photographed by the Galileo spacecraft. It is about the same size as our moon, but without major features such as mountain ranges or large impact craters.

Scientists believe that may be because its surface is covered by a liquid sea, crusted over by ice, which shows enormous moon sized cracks in this picture. An additional dedicated mission to Europa is being planned in about 2020, where a landing craft will see just how deep the outer crust is. If it isn't too deep, or an opening via a crack or fissure can be found, the plan is to inject a hydrobot into the liquid below to explore for life.