It was declared a World Natural Wonder in 2011. Only 600 people can visit it per day. You must purchase a permission pass from the government booking agency in advance. Often they are sold out for weeks, so don't wait too late. There are no exceptions, no pass, no entry. I have talked to a lot of people that missed visiting the Underground River because they could not get a government pass.

I took my passport (required) and got a trike down to the government office in Puerto Princesa . Two other people from the guesthouse went also. It was February 11. We wanted to get passes for February 13 (Friday the 13th). Fully booked. No passes available. But, they could fit me in on Feb 20. I planned to return to PP to fly out of Palawan Island on the 23rd, so I took the pass for the 20th to see the river cave. The other two people could not stay that long and so they did not get a pass to see the cave.

I made my planned trip to El Nido and returned to Puerto Princesa on February 19 in order to be ready for my Underground River trip.
 
The road to the cave is winding and bump and takes a couple of hours in the minivan. When we reached the place to get the boat to the Underground River, we were served lunch while we waited for our boat number to come up. It took a couple of hours for our number to turn up. In the meantime we browsed the gift and food stands and looked around the beach along the dock.

Our passes (given to us by the government office) was checked against our passports and we were loaded onto our small boats . This boat would only take us to the island where the Underground River was. After landing, we hiked a short distance into the forest until we reached the area of the Underground River.

There was another wait here while we again waited for our boat number. But, the time passed quickly because the area was shady and beautiful. We watched monkeys playing in the trees over our heads, and sometimes coming down to see what we had (that they might steal). 

Eventually our boat number came up and we piled for the trip into the underground cave. This boat was quite small. Everyone was given life vests and hard hats to wear.

The small boat took us inside the cave (maybe about 1/4 mile) where thousands of bats were hanging overhead. Ocasionally, water would drip on your head or arm from above. I hope it was water and not bat droppings! 

The water was clear and looked so refreshing . You could see fish swimming below. It was something to see. 

The roof of the cave was about 200 feet over your head. The person sitting in the front of the boat had a spotlight to control for us. The Filipino guide (that paddled the boat) would tell the person where to shine the light. Other than that, it was total darkness. The only sound was the voice of the boat guide as he paddled us silently though the cave.

The cave was too dark for photos, so there are none of the inside of the cave. The guide pointed out interesting rock formations. He spoke good English with a Filipino accent. It was enjoyable to listen to his pronunciations of the English words.