The watery centre of Europa
appears to be caused by the huge tidal
forces caused by orbiting a humongous lump of matter like Jupiter
Similar mechanisms are thought to cause the tremendous volcanism
on the nearby (rocky) moon of Io
The absence of any external power source (i.e. sunlight) to drive this biosphere was thought to be a problem until the discovery of black smokers a.k.a hydrothermal vents in the deep oceans of Earth.
If anybody has a very large drill, go and check it out.
Wrap up warm, though.
[Ed. note (Gz) - Minor spelling change for better integration 3/17/02]