Deep in the foothills of the Smoky Mountains, about 50 miles south of Knoxville, Tennessee, sits an wonder 140 feet below ground: the largest underground lake in America. The lake, which was discovered only decades ago, attracts about 2,000 visitors a day from all over the world. Chip Reid reports.

Some have survived for hundreds of millions of years, but many of the world’s freshwater megafauna—including sumo-sized stingrays, colossal catfish, giant turtles, and gargantuan salamanders—may soon find themselves on the brink of extinction, according to a new study published.

For the first time, researchers have quantified the global decline of freshwater megafauna—including fish, reptiles, amphibians, and mammals—and the results paint a grim picture. In four decades since 1970, the global populations of these freshwater giants have declined by almost 90 percent—twice as much as the loss of vertebrate populations on land or in the oceans.

Large fish species, such as sturgeons, salmons, and giant catfishes, are particularly threatened, with a 94 percent population decline. Most large freshwater reptile and many mammal species are also in trouble. The baiji, a Chinese river dolphin, is likely the first dolphin species driven to extinction by humans, and the Chinese paddlefish, which can grow 20 feet long, has not been seen in over a decade. Other species may be down to their last few individuals.

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