I have been to one, in New Zealand. It is part of a cave in the north island, south of Auckland. It was a fascinating and thrilling experience. At the roof of the underground river, you would see glow worms hanging off on the roof, they look small specs of greenish lights and so when you look up, they look like stars. These worms become moths without mouths and they live only for a few days (21 if my memory serves me right), they would mate and deposit their larvae onto the cave roof, then these moths die and the cycle rolls over again. The larvae are the glow worms.
This one in Puerto Princessa appears to be bigger and longer. She is 8kms long. Her current rank is 9th, so you can make her be part of the first 7 and perhaps one day you can even visit. I have been to Puerto Princessa, but not in the river, I have visited her satellite reef islands and walked on top of one of them. You can see clear water 30 feet down. Then little tropical fishes with loud colors, come around your toes and nibble at whatever they can - yes between your toes.