A bay a few hundred metres across, closed by a horseshoe of reef, where between May and November the tide traps plankton until the water turns to soup. Manta rays come in by the dozen — sometimes more than a hundred at once — wheeling in slow circles with their mouths open a metre below the surface. It is the largest known manta gathering anywhere. Entry is snorkel-only, with a guide and a daily cap on numbers, and whale sharks turn up too.