The Puerto Princesa Underground River runs eight kilometres through the inside of a limestone mountain on Palawan, in the Philippines, and comes out into the sea. Parts of it move through chambers sixty metres high. You reach it from the beach at Sabang, where narrow wooden outriggers take small groups along the coast. Jungle-topped grey rock walls the channel the whole way, and the boats tie up in a bright green cove inside it. A boardwalk runs from there to the cave mouth, and on it big monitor lizards lie flat on the warm sand while long-tailed macaques work the branches overhead. The macaques are bold enough to take anything loose out of your hands. Then you go in, wearing a hard hat and a life jacket, in a canoe the boatman paddles by himself.
He holds the lamp as well, so the only rock you see is whatever he points it at. The rest is dark, and the way to take the cave in is to listen. The clicking overhead is swiftlets finding their way through total blackness by sound, the same trick as the nine kinds of bat sharing the ceiling above them. The standard run turns back after about a kilometre and a half. For much of that you are floating on a thin skin of fresh water lying over seawater, which the tide pushes six kilometres into the mountain. Heavy rain flushes the salt back out, so the mix shifts with the season. Outside again, the cove is where people cool off and let their eyes readjust. Almost none of this system has actually been seen: in 2010 explorers found a whole second level above the one everyone paddles, with small waterfalls of its own. The same survey mapped a rock dome three hundred metres above the water, more channels, a deep pool and sea creatures living far inside. Most of the river has still never had a light on it.