Carlsbad Caverns in New Mexico is an underground wonderland. Subterranean features, embedded in several thousand feet of limestone, continue to grow drip by drip. Self guided tours take you in and out of chambers, along deep fissures and formations that take your breath away. All this created when ancient seas drained and continents shifted millions of years ago. Porous limestone was eaten away at fissures by decomposing marine life and sulfuric acid created by gas deposits far below the surface mixed with oxygen from the water. This is close to the long lengthy version in the brochure :)
750 feet below the surface lies the Big Room, a natural limestone chamber that is almost 4,000 feet long, 625 feet wide, and 255 feet high at the highest point. There is no concern about claustrophobia in this cave. Oops, I mean cavern. Like the square vs rectangle topic: all caverns are caves but the converse is not true. The cave is usually just one opening. However, the word cavern implies that there are a series of caves or cave chambers. The open spaces of caves connected by various passages is what cavern refers to.
Of the 119 caves only 3 are open to the public. Explorations still continue to find additional wonders.
This is one of the national parks to bring the kids or grandkids to.