"See the turtle of enormous girth, On his shell he holds the Earth."
While the Behemoth Tortoise doesn't necessarily hold the Earth, it can support a home or even a small village, depending on the size of it's shell. The settlers live in an almost symbiotic relationship with the tortoise, cleaning the shell and hunting some of the more troublesome parasites in exchange for a secure and mobile shelter.
Because of their security and fairly stable ecosystems, these shells are prime real estate, leading to some gruesome battles for squatter's rights. This has given rise to that famous children's game, "King of the Shell."