Most exciting of all: Europa, Ganymede and Callisto all almost certainly possess underground oceans of liquid water.

A diagram showing a cutaway of Europa.
Warmth from Europa’s interior and tidal energy from Jupiter likely maintain a massive liquid ocean beneath the moon’s icy surface. NASA/JPL-Caltech/Michael Carroll
Ocean worlds
Europa, Ganymede and Callisto have chilly surfaces that are hundreds of degrees below zero. At these temperatures, ice behaves like solid rock.