What is a subsurface ocean in the Solar System?

Scientists think several bodies in the outer Solar System may have oceans sealed under icy crusts, including Europa, Enceladus, Ganymede, and possibly Pluto. These oceans can be kept liquid by tidal heating, radioactive decay, or internal chemistry. They are scientifically important because liquid water is a key ingredient for life as we know it. Evidence for hidden oceans often comes from magnetic data, surface fractures, and plumes of material.