There are four mighty World Elephants that stand upon the carapace of Great A'Tuin the World Turtle, and upon whose massive shoulders revolves the Discworld itself. Their names are Tubul, Jerakeen, Berilia and Great T'Phon. There is a legend that once there were five, but one lost its footing, entered an unstable orbit and came hurtling through space, smashing into the Disc and causing the mountains to rise. Its blood and bones were not merely blood and bone but mercury and gold (the Dwarfs have some very good mythology for a race that holds that it is not religious), which is why they are found underground, and its body over the years became the huge fat deposits under and around Überwald.
The legend and a great deal of commotion is to be found in The Fifth Elephant.