There are a few theories as to how Saturn's moons formed. Space.com notes that some scientists believe that there were actually two Titan-sized moons at one point, and that the other one broke apart for some reason, the debris forming the rest of the planet's satellites. A contrary theory postulates that Saturn's orbit was home to several fairly large moons, two of which ended up fusing together to form Titan.
To help try and solve these and other mysteries of the gigantic moon, the Cassini mission actually landed a probe to land on the surface of Titan in 2005, and what they learned about it was truly mind-blowing, enough for NASA to call Titan a "remarkably Earth-like world." They discovered lakes and vast seas of liquid methane, and evidence suggested that the moon has a large liquid sea of water and ammonia beneath its surface, as well. Scientists have also observed sinkholes on its surface. But despite the wealth of information the Cassini mission provided, we still have so much more to learn about Saturn's numerous and diverse moons.
