From the World Heritage inscription:

This park features a spectacular limestone karst landscape with an underground river. One of the river’s distinguishing features is that it emerges directly into the sea, and its lower portion is subject to tidal influences. The area also represents a significant habitat for biodiversity conservation. The site contains a full ‘mountain-to-sea’ ecosystem and has some of the most important forests in Asia.

The underground river is really pretty cool. The river goes into the mountain about 6km, but the tours only go in about 1km. The inside of the cave has tons of stalactite and stalagmite formations, most of which are named after things they look like, which usually requires a lot of imagination. Outside the cave, I saw huge monitor lizards and macaque monkeys. The underground river on Palawan is the longest navigable underground river in the world.