Hanifaru Bay is a dead-end pocket of reef in Baa Atoll, in the Maldives, no larger than a football pitch, and for most of the year nothing at all happens in it. What fills it is plankton, pushed in by the tide with nowhere onward to go. Reef mantas will not bother with thin water; they start feeding together only once the plankton passes a threshold worth the effort, more than twice what an animal needs simply to get by. Past that point they arrive by the dozen, up to around two hundred, and stack into slow-turning columns just under the surface, mouths held open. Researchers here have catalogued eight separate ways they feed, from a single animal barrel-rolling alone to the mass vortex the bay is known for. The same plankton brings whale sharks, and this is the country's second most important gathering place for them, though far less predictably. Outside the feeding zone the coral gardens hold stingrays, turtles and schooling reef fish, with blacktip reef sharks crossing white sand in water barely waist deep.
None of it runs to a timetable. No boat leaves until someone in the water has confirmed that the mantas have gathered, so a stay can end without a single call. When the call does come, there are no tanks, because bubbles break up a feed, and nobody chases anything, which gets you nowhere in any case. You slip in at the edge of the group and let the current carry you, three metres off every animal, with rangers in the water watching how it goes. As the mantas roll to funnel the thickest plankton, their white bellies come up at you one after another. Every one of those bellies carries its own scatter of dark spots, fixed for life, so a single square-on photograph is enough to name the animal. More than six thousand reef mantas are on the Maldivian register, a great many of them first logged right here. The belly turning over your head is almost certainly already in that file, under a number.