Hạ Long Bay is some 1,600 limestone towers standing out of the green water of the Gulf of Tonkin in northern Vietnam, and for generations people lived their whole lives among them without moving ashore. The name means descending dragon. In the story a mother dragon and her children spat pearls and jade into the sea to wall off an invading fleet, and the jewels hardened into these islands. What the rock did was slower. Rain dissolved a limestone plateau into caves and shafts over millions of years, then the sea rose and drowned the lowest of them. So many of the openings here sit right at the waterline. A kayak goes under the low arches into lagoons ringed by sheer walls, where the water turns glassy and every sound comes back off the rock. The largest cave is Sung Sot, the Surprise, inside Bo Hon Island: a climb up from the boat, then chamber after floodlit chamber of stalactites, each bigger than the narrow entrance led you to expect.
A wooden junk takes you out through the straits, and the bay gives up a new tower at every bend. Staying the night aboard is what separates this from a day out. The sun drops behind the towers and the water goes copper, the boat lies at anchor under the stars, and at first light you come out on deck into a low mist wrapped round the islands. Under the cliffs are the rafts. For generations fishing families lived afloat here, in villages of wooden houses lashed together and moored in the shelter of the rock, with fish pens alongside and a school floating among them. That ended in 2014, when more than five hundred people were resettled on the mainland. A bamboo boat still goes through what is left, past the pens and the old floating school, and you are being rowed through the last of a life that was lived entirely on the water.