Carlsbad Caverns' Big Room

Carlsbad Caverns National Park in southeastern New Mexico contains the largest natural cave chamber in North America. The Big Room is roughly 4,000 feet long, 625 feet wide, and 255 feet high at its tallest point, large enough to hold six football fields. The caverns were discovered by a 16-year-old cowboy named James Larkin White around 1898, who descended into the cave by torch and explored its chambers for years before anyone believed his descriptions of what was down there. The cave also houses a colony of between 400,000 and 1 million Mexican free-tailed bats that emerge at dusk in a column visible from miles away.

Source: Wikipedia