Limestone formations on the exit from the 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. Boats, waiting for turists, are visible near the exit.