The Cracking Fells were the only obvious connection in the Shire to the subterranean world of the Under-deeps. A potent source of supernatural power lay almost three miles below the surface of the fells. Whether it was sentient or construct, male, female, or subtle device, none could say. The Elves living in the Fells communicated with high-pitched whistles and yodels that sometimes caused them to be taken for supernatural beings. The creatures known as Rockhoppers were goblins of the Under-deeps. They appeared roughly every 10-100 years, or whenever the Forces of Darkness were unusually active. Other creatures that might appear in the Fells included wraiths, giant insects, killer bats, and cave worms.
The decline of magic in the later Third Age meant intrusions from the Underdeeps became even rarer in the Cracking Fells. Sauron's resurgence in the 30th century of the Third Age brought up new ones: ghosts, goblins, abominations, and the occasional giant sheep-killing earth-worm. Local Hobbits fought back against these haunts using fire and knowledge scoured from old books. The better folk of the Westfarthing sought to suppress the "ugly rumors" about these unpleasantries, so Frodo left the Shire not realizing that Sauron's influence was popping up practically in his backyard. The truth came out after Sharkey's overthrow ; Saruman and his apprentices had attempted to summon supernatural forces from the Cracking Fells to replace the powers Gandalf had "stolen "from the White Wizard at Orthanc. Expeditions, including at least one Hobbit and one female Dunlending who had studied under Saruman, were mounted to go into the Underdeeps and eliminate the problem permanently.
- MERP:the Shire