It is a rocky cave system which is home to the Fraggles, as well as various other creatures such as the Gorgs and Doozers.
Fraggle Rock is a children’s television series that aired on HBO from 1983 to 1987. The series was created by Jim Henson and revolves around the Fraggles, a race of small, furry creatures who live in caves and have their own unique culture. The Fraggles live in a place called Fraggle Rock, which is hidden away from the human world. The series follows the adventures of the Fraggles as they explore their world and interact with the other creatures who live there.