A picture of the surface of Europa, taken by the Galileo spacecraft. The dark areas are a clue that there might be liquid underneath.
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Image courtesy of NASA
The surface of Europa shows many signs that there may be an ocean under it's icy surface. In this picture, the dimples, shown in red, may be a sign of material falling into a water under the ice. Or they be caused by material rising up from moving water underneath.
Other signs of an ocean under the surface of Europa include: