The Puerto Princesa Underground River is a river on the island of Palawan in the Philippines that flows for eight kilometres through the inside of a limestone mountain and out into the sea, through chambers up to sixty metres high, and the first part of it is paddled in a canoe. The trip starts on the beach at Sabang, where narrow wooden outriggers carry small groups along the coast to a lagoon at the foot of the limestone cliffs; a boardwalk through the forest leads to the cave mouth, and on it big monitor lizards lie flat on the warm sand and long-tailed macaques work the branches overhead. Then you put on a hard hat and a life jacket and sit in a canoe that the boatman paddles alone. He holds the lamp too, so in the dark you see only the piece of rock he points it at: a chamber the size of a cathedral, a stalactite shaped like a saint, the roof crowded with bats and swiftlets that find their way in the blackness by clicking as they fly.
The tourist route goes about a kilometre and a half in and turns back, and the river continues for six more that only researchers see. For its final kilometres it is barely fresh water at all: the tide pushes the sea so far into the mountain that salt water and river water lie layered one over the other, and the river's mouth opens straight onto the beach. In 2010 explorers found a second level hidden above the one everyone paddles, with its own small waterfalls, and the mountain above holds untouched forest from cave mouth to summit. The park around the river is one of the few places in the world where a river, a cave, a rainforest and a reef sit in one protected block. Daily numbers are capped, so the trip is planned rather than dropped into. The Underground River is a river you enter through the side of a mountain, and it goes on long after the lamp turns back.