Under a swath of Kentucky hills and hollows is a limestone labyrinth that became the heartland of a national park. The surface of Mammoth Cave National Park encompasses about 80 square miles.

Where you now see ravens soaring over a stark landscape, leathery-winged pterosaurs once glided over rivers teeming with armor-scaled fish and giant ... was designated a national monument in ...

Robert F. Kennedy, Jr., calls the Great Bear Rainforest "the planet's last large expanse of coastal temperate rain forest." Stretching for more than 250 miles along the coast of British Columbia ...