From: https://www.somethingawful.com/news/really-inconvenient-truths/

Welcome to Hell

The largest subterranean cavern is believed to be the Pacific Cavity. Located almost two miles beneath the surface of the Pacific, this cavern system was detected by ground penetrating radar in 2000 and covers at least 48 square miles. Some scientists believe it was formed as an immense methane pocket vented into the ocean and that much of it remains dry.

Caveat: There is a 100% chance that whatever is living inside the Pacific Cavity is scary as shit.