The park has a limestone karst mountain landscape. St. Pauls Underground River Cave is more than 24 km (15 mi) long and contains an 8.2 km (5.1 mi) long underground section of the Cabayugan River. The river winds through the cave before flowing directly into the West Philippine Sea [2] and is navigable by boat up to 4.3 km (2.7 mi) in from the sea. The cave includes major formations of stalactites and stalagmites, and several large chambers, including the 360-meter-long Italian's Chamber with approximate 2.5 million square meters volume. It is one of the largest cave rooms in the world.[3] The lower portion of the river up to 6 km from the sea, is subject to tidal influences. Until the 2007 discovery of an underground river in Mexico's Yucatán Peninsula,[4] the Puerto Princesa Subterranean River was reputed to be the world's longest underground river.
The area also represents a habitat for biodiversity conservation. The site contains a full mountain-to-the-sea ecosystem and has some of the most important forests in Asia. It was inscribed by UNESCO as a World Heritage Site on December 4, 1999.