The St. Paul Subterranean Underground River has recently been declared as one of the New 7 Wonders of Nature, due to its unique natural phenomenon.
Below the St. Paul mountains lie an 8.2 km long underground river that flows directly to the sea. The lower half of the river is brackish and subject to the oceans tide.
It's hard to fully envision the experience unless you go and visit it. The river is pitch-black dark and is only lighted by the small torches from the boats. Permits are required ahead of time and in recent months, there seem to be a 30 day advance booking required for tours. You can still go directly to Puerto Princesa in Palawan, which is about an hour flight from Manila, but there is a chance you will not be able to get a permit, so better to book ahead. You have to travel 2 hours by land from Puerto Princesa (where the airport is) to Sabang Beach, then hop on a 15-minute motorized banca (outrigger boat) to the entrance of the underground river (shown in the photo). The actual river tour starts there as you board a small paddle boat and enter the river. The boatman paddles your boat at the same time, be your guide inside the river and showing you the formations found inside the cave. The tour is about 45 min and it only covers less than 2km of the river. If you are a scientist or part of the media wanting to explore more of the river, you have to get a special permit to enter the inner portion of the river, which is now closed to the public.
Note: This highlight also appears in AFAR Website