The Puerta Princesa Underground River

THE UNDERGROUND RIVER (Puerto Princesa, Palawan, The Philippines)

Until 2007 considered the world's longest underground river, the one in Puerto Princesa (Palawan) in the Philippines contains waterfalls, a 980-foot-high dome, rock formations, large bats, and more. The cave runs 15 miles; 5.1 miles of that are river, running into the West Philippine Sea (the first 2.7 miles are navigable by boat!). Numerous plant and animal species occupy the cave and the park it's in.

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(Image courtesy Wikimedia Commons; CTTO)