The Big Room is the point. It is a single limestone chamber almost 4,000 feet long, 625 feet wide and 255 feet high at its tallest, the largest cave chamber in North America and the 32nd largest in the world, and you can walk a loop around the floor of it.
The rock it is cut into was a reef. Around 250 million years ago this part of southeastern New Mexico was the coastline of an inland sea, and the Capitan Reef built up along it, though not from corals the way a modern reef does. The Permian reef was made by bryozoans, sponges and other organisms. The sea eventually went, the reef was buried, and much later it was lifted into the Guadalupe Mountains and dissolved out from within.
How it was dissolved is the unusual part. Most limestone caves are cut by rainwater carrying carbon dioxide. Carlsbad was hollowed out largely by sulphuric acid, generated where hydrogen sulphide rising from the oil and gas reservoirs below met oxygen-rich groundwater. That is a far more aggressive process, and it accounts for the sheer volume of rock removed and for the gypsum blocks left behind.
You can go in two ways: down the natural entrance on foot, a switchback descent of about 750 feet that takes roughly an hour and is by far the better arrival, or by lift from the visitor centre directly to the Big Room.
The park became a national park in 1930 and a World Heritage Site in 1995. Two thirds of it is designated wilderness, and it holds two entries on the National Register of Historic Places. Seventeen species of bat live in the park, including many Mexican free-tailed bats, and a section of the cave is still called the Bat Cave, having been mined for guano in the early twentieth century. The free-tailed population is thought once to have numbered in the millions and has declined sharply since, for reasons that are still not settled.
Other places on the map within a couple of hours.
