Limestone formations in the cave, that features a limestone karst mountain landscape with an 8.2 km. navigable underground river, supposedly the longest navigable underground river in the world. 50 kilometers north of the city of Puerto Princesa, Palawan, Philippines. February. a World Heritage Site in the the Puerto Princesa Subterranean River National Park, in Saint Paul Mountain Range on the northern coast of the island. A distinguishing feature of the river is that it winds through a cave before flowing directly into the South China Sea.