Callisto

It is the 3rd largest moon in the solar system, behind its neighbor Ganymede and Titan. It has one of, if not, the most cratered surface in the entire solar system.

History
Callisto formed from a disk of gas gathered by Jupiter's gravity. It formed along with the other Galilean Moons around 4.503 billion years ago.

Ocean
Like Ganymede, Callisto has an icy-rocky surface. And like Europa and Ganymede, it has an ocean below its surface.