One of the surprises of the outer Solar System has been just how geologically active it is. The giant planets there put their moons through some rather extreme gravitational strains, and the internal friction is actually capable of melting some of their internal material. In many cases, this is apparent by the young surfaces of moons like Europa the relative paucity of craters suggests its surface is regularly remodeled.
But the two most spectacular examples are Io and Enceladus. Jupiter's moon Io is the most volcanically active body in the entire Solar System, with Lava lakes and volcanic plains, along with 300km high plumes of liquid sulfur. On Enceladus, it's geysers of liquid water, which rapidly freezes in the cold, atmosphere-free environment.
This past week, scientists came out with papers describing the internal liquid reservoirs that feed these eruptive activities. And for both these moons, the evidence suggests that the reservoir is global in Enceladus' case, the crust and core of the moon are likely to be disconnected.