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Though this is Baker’s most conventional story, he is still the 21st century’s best societal spelunker, with a filmography that plays like an underground study of modern economics.

Experts suggest it could have arrived, for example, on the boot of a spelunker who trekked it in from Europe or Asia.

Known as a destination for spelunkers and hikers, Maquoketa Caves State Park has more caves than any other state park in Iowa.

At the entrance to Tenglong's lower cave, where the Qing River surges toward the Yangtze River, local spelunkers geared up on a recent day, fastening helmets, snaking ropes over their shoulders, packing waterproof bags.

In 1961, a group of Milanese spelunkers traveled to southern Italy to map a deep cave in the floor of a remote mountain valley.