Vava'u is a maze of some fifty limestone islands in the north of Tonga, and each winter about two thousand humpback whales come into its sheltered channels to court and calve. It is one of a handful of places on Earth where the law lets you into the water with them. The rules are strict: a licensed operator, a guide in the water, never more than four swimmers, who slip in quietly and wait, because the whale decides whether to stay. When one does, a mother hangs motionless in the blue a few metres off while her newborn spirals up to breathe and comes back to look at you. If a male is singing nearby, the sound arrives as a pressure in your chest rather than as noise.
The limestone around all this is full of caves at the waterline. A kayak or small boat goes straight into Swallows Cave through an opening on the flank of Kapa Island. Around midday on a calm, sunny day the light comes up through the water and turns the whole chamber electric blue, with small fish below you and swiftlets nesting on the high walls. Mariner's Cave has no opening above water at all. You take a breath and follow the guide down through a submerged tunnel into a sealed pocket of air. The trapped air compresses with every swell, so a fine mist forms and vanishes in the half-light. Tongans tell that a young chief once hid a girl promised to a rival in there, feeding her in secret until he could carry her off by canoe to Fiji. The town is Neiafu, on the Port of Refuge, an inlet so well enclosed that boats ride out the whole cyclone season inside it. Only male humpbacks sing, and every male in a population sings the same tune. Every few years a new one sweeps through and replaces it, starting off eastern Australia and passing east through Tonga toward French Polynesia, so what you felt in your chest was one verse of something crossing an ocean.