Kayangan Lake sits in a bowl of limestone on Coron Island, in the Palawan province of the Philippines, and the water in it is part rain and part sea. About seventy per cent of it fell as rain; the rest crept in through the rock from the ocean outside, and the two barely mix. You feel that as you swim. The temperature reaches you in bands, cool and then suddenly warmer, and a metre or two down there is a faint blur where the fresh water lies on the salt. Hardly anything lives in that mixture, which is exactly why you can see so far. With your face in the water, the cliff you have just climbed carries on beneath you, ledge after ledge, until it fades into blue.
Some 150 rock steps, polished smooth by everyone who came before you, rise from the cove where the outrigger boats moor. Halfway up, a deck opens on the picture Coron is sold by: green water, a scatter of anchored bangkas, limestone spires behind them. Then the path tips over and drops to a wooden walkway along the shore, the only thing built on the lake, where voices come back off the karst walls and the bowl goes quiet between boatloads. Coron Island is Tagbanua ancestral domain, title granted in 2002, and the lake is one of thirteen the Tagbanua hold sacred. They did not simply open it: in 2001 their elders performed a ritual called uliwansag to ask the lake's spirits to allow outsiders in, and only two of the thirteen are open now. The name comes from kayangag, the Tagbanua word for the red-vented cockatoo, a bird down to a few hundred in the wild and unlikely now to be seen anywhere near the lake it named. The other of the two, Barracuda, looks like this lake's twin from the surface. But a diver sinking past fourteen metres crosses into brine near 38°C in a single fin kick.