Cassini made the incredible discovery of liquid water on two of Saturn's moons, Enceladus and Titan. Even more fascinating was Enceladus' jets.
Discovered on the ice moon in 2005, these jets are geysers of water vapour and ice particles from a liquid ocean beneath Enceladus' icy crust. Cassini actually flew through these jets and took measurements, finding molecular hydrogen.
This makes Enceladus a promising place to look for extraterrestrial life. Although there's little light there, researchers believe there may be hydrothermal vents - which in oceans on Earth, down in the dark, are teeming with life.