Hạ Long Bay is a stretch of the Gulf of Tonkin in northern Vietnam where some 1,600 limestone islands and towers stand in green water, sailed among on wooden boats and paddled through by kayak into lagoons sealed inside the rock. The name means descending dragon: in the legend a mother dragon and her children hurled pearls and jade into the sea to stop an invading fleet, and the jewels became the islands. What the legend describes, the rock confirms. The towers are what is left of a limestone plateau that the sea and rain have been dissolving for millions of years, and the bay is full of the holes they made: caves, arches and hollow islands. Sung Sot, the Surprise Cave, is the largest, hidden inside Bo Hon Island, a climb up from the water into chambers big enough to hold a crowd, lit by coloured lights and dripping from the roof. Under lower arches a kayak slips through gaps the sea has cut at the waterline and comes out in a lagoon ringed by sheer walls, closed off from the open bay and silent.
The way to see it is to sleep on it. A wooden junk goes out from the mainland and spends a night or two among the islands, and from the deck the water gives up a new tower at every bend as the boat slips through narrow straits, with the karst going grey and then black as the light fails. For generations fishing families lived their whole lives afloat here, in villages of rafted wooden houses moored under the cliffs; almost all were moved to the mainland in 2014, and the few floating villages that remain are paddled through with their former residents at the oars. The bay is busiest and haziest in summer; October and November bring the clearest air of the year, the karst reading sharp to the horizon and the sea still warm enough to swim in. Hạ Long Bay is the dragon's jewels in a single view, and no two boats see the same ones.