Vava'u is a group of some fifty islands in the north of Tonga, in the South Pacific, a maze of raised coral islands and channels where about two thousand humpback whales come each year to calve and court, and it is one of the few places on Earth where the law lets you swim beside them. From July to October the whales fill the warm channels, mothers with newborn calves resting in the sheltered water and males singing; only male humpbacks sing, and every male in a population sings the same slowly shifting song, which you can feel through your body when one passes in the water. Licensed boats take groups of no more than four swimmers at a time, and the method is to slip quietly into the blue with a guide and wait, since the whale chooses whether to stay; when it does, a mother and calf hang a few metres away in water so clear that the whole animal is visible, and the calf comes up to look.
The islands themselves are limestone, riddled with caves at the waterline. Swallows Cave, on the flank of Kapa Island, is entered by kayak or small boat straight through a sea-level opening into a chamber where the light falls through the water in shafts; Mariner's Cave has no opening above the water at all, and you take a breath and follow the guide down through a submerged tunnel to surface inside a hidden air pocket sealed in the rock, where the air fogs and clears with every swell. The cave carries a Tongan love story of a young chief who hid a girl promised to a rival inside it, feeding her in secret until he could spirit her away by canoe to Fiji. The town is Neiafu, on the Port of Refuge, a turquoise inlet so well enclosed that yachts stay inside it through the whole cyclone season, one of the safest natural harbours in the Pacific. Vava'u is a nursery for whales, and the whales let people into the water with them.