Entry to a cave that features a limestone karst mountain landscape with an 8.2 km. navigable underground river . 50 kilometers north of the city of Puerto Princesa, Palawan, Philippines. February. supposedly the longest navigable underground river in the world - 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.