Billingsley tells David that the cause of the cave-in at the China Pit is rumored to have been caused by Tommyknockers.
"There are all kinds of stories about it. One is that they dug up a waisin, a kind of ancient earth-spirit, and it tore the mine down. Another is that they made the tommyknockers mad.”
“What’re tommyknockers?” David asked.
“Troublemakers,” Johnny said. “The underground version of gremlins.”
- Part III / Chapter 3 / Section 2