Europa is one of many moons which orbit the planet Jupiter. For years now we have known that Europa most likely has an ocean of water lurking deep under the frozen icy surface. The surface of Europa is almost all water ice. It has cracks on the surface resembling many found here on Earth. How does it get warm enough for water to exist under the surface? Tidal forces from Jupiter and other moons are causing Europa to flex, generating interior warmth.
Europa has been a prime focus for life as well, since the 1980s. Arthur C. Clarke in "2010" used it. If water exists on Europa and it most likely does, this would also change our focus on where to look for life in the universe and how it may come to exist.
New data from the Galileo spacecraft have found blocks of ice on the surface that suggest an interaction between the icy surface and a sub-surface lake of water.