Although the world has thousands of caves, many of which have barely been explored, this article will highlight four key caves that any caver would agree are among the best in the world. These are the Mammoth Cave in Kentucky (the world's largest cave system), the Carlsbad Caverns in southeast New Mexico (among the largest and best-decorated caves in North America), Lechuguilla Cave in southeast New Mexico (recently discovered, 5th longest cave in the world, deepest in the United States, among the best preserved), and the Crystal Cave of the Giants, part of Naica Mine in northern Mexico.
Probably the most famous cave in the world, the Mammoth Cave system is also the longest known cave system in the world, including more than 360 miles (580 kilometers) of passageway. The Mammoth Cave is one of only a few caves to be listed as a World Heritage Site, and is the only cave to be listed due to its natural qualities rather than human-built statues or paintings found there. The Mammoth Cave is located in Edmonson County in central Kentucky, where it attracts hundreds of thousands of visitors per year. The cave features huge chambers and spectacular formations, including "Frozen Niagara," a "waterfall" of cave formations.
Another famous cave is in New Mexico, known as the Carlsbad Caverns. Its central feature is the Big Room or Hall of the Giants, with a floor space of 33,210 square meters (357,459 sq ft). Featuring numerous scenic rooms, including the Green Lake Room, the Bifrost Room, King's Palace, Lake of the Clouds, Hall of the White Giant, and the Queen's Room, widely regarded as the most spectacular room in the cave. During the Cold War, the Carlsbad Caverns were evaluated as a possible fallout shelter for many thousands of people.
An amazing cave recently uncovered, located near the Carlsbad Caverns, is the Lechuguilla Cave, only accessible to approved scientific researchers, survey and exploration teams, and National Park Service management-related trips. This cave is the fifth-longest in the world at 120 miles (193 km) long and the deepest in the continental United States at 1,604 feet (489 meters). It is known for its pristine condition, unusual geology, and rare formations, some of which are unique in the world. One room, the Chandelier Ballroom, has gypsum formations as large as 60 ft (18 m) or more, and individual chandeliers that extend 20 ft (6 m) from the ceiling.
The Crystal Cave of the Giants is among the most spectacular caves in the world, but it is off-limits to practically everyone. This cave contains numerous natural selenite crystals which are the largest natural crystals ever found. Some are 30 ft (9 m) long and weigh 50 tonnes. Deep under the ground, this cave has temperatures that approach 130 degrees F (54 degrees C), meaning that all entrants must wear special protective suits. Because of these harsh conditions, this cave has barely been explored.