The Puerto Princesa underground river

Floating on a few centimetres of fresh water over seawater, underground

The Puerto Princesa Subterranean River runs through the karst of the Saint Paul Mountain Range on the west coast of Palawan, about 76 km northwest of Puerto Princesa and 360 km southwest of Manila. An 8.2 km stretch of the Cabayugan River travels underground through a cave system at least 32 km long and comes out directly into the South China Sea. It was long reputed to be the world's longest underground river, until a longer one was found in Mexico's Yucatán Peninsula in 2007, and it is now counted second.

Because the river flows straight into the sea, its brackish lower half is subject to the tide. Salt water reaches about 6 km into the caves, with the fresh water riding on top in a layer only a few centimetres thick, and heavy rain floods the seawater out for a time. Boats can go in about 4.5 km by UNESCO's figure, past chambers as much as 120 metres wide and 60 metres high; the Italian's Chamber, 360 metres long, is one of the largest cave rooms in the world. In 2010 environmentalists and geologists found that the river has a second floor, with small waterfalls, and a cave dome 300 metres above the water. The deeper areas are almost impossible to explore for lack of oxygen. Nine species of bat, two of swiftlet and a whip spider live in the cave.

The park takes in the entire watershed, about 22,202 hectares, a full mountain-to-sea ecosystem with eight forest formations from mangrove and beach forest up to montane forest. Researchers have identified more than 800 plant species, and 165 of the 252 bird species known on Palawan, including all 15 of its endemic birds, among them the Palawan hornbill. Some 30 mammals have been recorded, from long-tailed macaques feeding along the shore at low tide to the bearcat and the Palawan stink badger, and dugongs feed in the coastal waters.

The park was the first in the Philippines to be devolved to and managed by a local government, and the city of Puerto Princesa has run it since 1992. UNESCO inscribed it on 4 December 1999. In 2012 it was confirmed as one of the New 7 Wonders of Nature, in a vote that allowed repeated voting and that the Philippine president urged the country's cellphone subscribers to join by text, and it became a Ramsar Wetland Site the same year. Most visitors come by road to the seaside village of Sabang and go on by boat.