Christmas Island is a jungle-covered limestone plateau in the Indian Ocean, an Australian territory 350 kilometres south of Java, where tens of millions of red crabs march out of the forest to the sea at the start of each wet season, one of the great animal migrations on Earth, and where the largest land crab in the world walks the roads all year. The island is the tip of an undersea mountain, rimmed with cliffs and terraces and covered in rainforest, and its red crabs live in burrows on the forest floor, keeping it unusually open and bare by eating the fallen leaves, seeds and seedlings; with the first rains, October to December, they stream out by the million, carpeting roads, gardens and forest floor in scarlet, all moving the same way toward the coast, where the males dig burrows on the shore terraces, the females follow, and on a night of the last quarter moon they release their eggs into the sea in a red tide at the water's edge. The island builds bridges and closes roads for them, and juvenile whale sharks gather offshore from November to March to feed on the billions of larvae washed out by the spawn.
The robber crab, or coconut crab, is the other giant, the largest land invertebrate on Earth, up to a metre across and strong enough to crack a coconut in its claws, armoured in deep blue or rust red and lumbering along the forest tracks unbothered by people. Raised boardwalks wind through the Dales, a chain of spring-fed rainforest valleys on the western side, to the mossy cascade of Hugh's Dale waterfall, and along the rugged southern coast sea caves funnel the swell up through vents in the limestone and launch spray several metres into the air with a boom. Seabirds fill the cliffs, among them the golden bosunbird, a slender tropicbird with a golden tail that nests almost only here. Christmas Island is a rock in the ocean run by crabs, and once a year they take the roads.