Hanifaru Bay is a pocket of reef the size of a football pitch in Baa atoll, in the Maldives, where the tides of the southwest monsoon trap plankton against a dead-end channel and draw up to two hundred reef manta rays into slow-spinning columns just below the surface, the largest known gathering of the species on Earth, snorkelled by permit in a UNESCO biosphere reserve. The bay is a cul-de-sac in the reef where the incoming tide meets the current from the lagoon and the plankton has nowhere to go, and mantas ignore thin plankton; when the concentration passes the threshold, from May to November and strongest around the new and full moon, dozens of them arrive and stack into a cyclone, circling nose to tail with mouths open to filter the water, and barrel-rolling through the densest patches so that their white bellies turn straight up at the snorkellers above. Every reef manta carries its own pattern of dark spots on its underside, and the researchers of the Manta Trust, who have worked the bay since 2005, identify individuals from photographs, so that visitors' pictures go into the database.
Hanifaru is a marine protected area, not open water. Diving is banned inside it, boats are limited, and the rangers on the water decide each day whether and when snorkellers go in, with a fee and a guide, so that a visit is a morning of waiting on the boat for the mantas to gather and a short window inside the bay. The same plankton also draws whale sharks, and the bay is the second most important gathering site for them in the Maldives after South Ari atoll; just outside the feeding zone the coral gardens shelter stingrays, schooling reef fish and turtles at a gentler pace. The bay is reached from the resorts and guesthouses of Baa atoll, half an hour by boat from most. Hanifaru is a trap for plankton that the mantas learned, and the cyclone is what they do with it.