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The Puerto Princesa Subterranean River in Palawan is an internationally recognized karst mountain landscape containing a navigable, 8.2-kilometer-long underground waterway.
As rainwater absorbs atmospheric carbon dioxide, it turns weakly acidic and gradually dissolves deep limestone formations over millions of years, carving immense subterranean caverns and cathedral-like chambers.
Because the lower half of the river is directly subject to oceanic tides, it creates an unusual brackish aquatic zone underground, fostering specialized bat colonies, blind cave crabs, and unique mineral formations that rely entirely on nutrients swept in from the sea.
UNESCO World Heritage Centre