The Liquid Ocean: There is strong evidence suggesting that underneath the its icy shell, Europa has a liquid ocean. This ocean is estimated to be about 100km deep and to contain two times the water in Earth's oceans. This ocean was first theorized after looking at modules of the tidal heating on Europa due to Jupiter's gravitational force. These modules suggested that these tidal forces can generate enough heat to sustain a subsurface ocean. Later on, better images of Europa's surface offered more hints to subsurface ocean. The shell of Europa has few impact craters, suggesting that its surface is young, only 20-100 million years old (young in geological time scales). This suggests that the surface is active and changing. Also, Europa has many strange geological features, including large bands, cracks, ridges, spots, domes and chaotic areas. Theses features, along with the shapes of the craters, strongly imply an active shell that is flowing on a liquid ocean. Europa's surface features - NASA A  model of Europa's ocean - NASA All of these are strong clues, but the most substantial piece of evidence for a subsurface ocean came from the Galileo spacecraft's finding of an induced magnetic field. Because of this ocean, Europa is a prime candidate for extraterrestrial life!