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Mammoth Cave owes its name to its size, not to relics of long-gone mammoths. Carved from limestone hills in Kentucky by trickles of acidic water over millions of years, the cave has some 330 miles (530km) of passages, possibly more, on five levels.
Native Americans knew this fantastic underground world some 4000 years ago - stone spearheads and burial remains have been found there. The earliest cavers lit the way with cane reed torches. Visitors today can view some of the scenery with electric light. It varies from vast halls such as the Rotunda, as spacious as New York's Grand Central Station, to narrow passages such as Fat Man's Misery, only 18in (460mm) wide.
Boat trippers on the Echo River, 360ft (110m) below ground, can test its resonant echoes. Frozen Niagara is a series of chambers where colourful stalagmites and stalactites look like waterfalls that turn to stone.
European settlers first came to Mammoth Cave in the 1790s, and during the war of 1812 between the US and Britain, its nitrate deposits were mined to make gunpowder. In 1843 the cave was briefly and unsuccessfully used as a tuberculosis hospital because it is around 12C (54F).
By then it had become a tourist attraction; its first guide and explorer was a slave named Stephen Bishop, who lit the way with a kerosene lantern. Now the cave is part of an 80sq mile (207km) national park, with guided underground tours.
