Aitutaki is a lagoon in the Cook Islands, a triangle of coral reef twelve kilometres a side, and the water inside it is shallow enough to wade across much of it. Fifteen small sand islands, the motu, are scattered around the rim. The day here is spent crossing the water rather than sitting beside it. A boat works from motu to motu, and the colour changes under the hull between deep channel and bright sand flat, pale green to a turquoise so strong it looks lit. Where the water barely reaches your knees you step out and walk to the next island across the flats. Most days finish at Tapuaetai, One Foot Island, whose name means one footprint. A father and son were caught fishing in forbidden waters, which carried a death sentence. On the motu the father hid the boy and then stepped carefully inside his son's own prints, so the warriors who followed read one set of tracks in the sand.
Between the islands the boat drifts over coral heads where green turtles graze and shoals of trevally and parrotfish pass close. Much of the lagoon is chest-deep and glass-clear, so you float above the reef without losing sight of the sand below you. Outside the reef the floor drops away. Between July and October humpback whales come up from Antarctica into that deep water, and small boats run out through the passage to find them. Swimmers go in a few at a time and then stay put, because it is the whale that decides how close to come. One climb takes in the whole of it. Maungapu, 124 metres, is what is left of the old volcano, and a steep half-hour path goes up it. From the top the reef line, the motu and the deep channels lie out in every shade of blue. Aitutaki says it did not grow that summit but carried it home from Rarotonga after a victory, and Raemaru, back on Rarotonga, has had a flat top ever since.