One of the world's deepest caves was carved from below by acid, not by water dripping down

Lechuguilla Cave in New Mexico stretches over 150 miles of mapped passageway, but it wasn't formed the usual way, by rainwater slowly dissolving rock from above. Hydrogen sulfide rising from nearby oil deposits reacted with groundwater to form sulfuric acid, which ate away at the limestone from underneath, carving vast chambers and rare formations found almost nowhere else on Earth.

— Carlsbad Caverns National Park, explored from 1986, Lechuguilla Cave — Speleogenesis studies