Mosquito Bay on the island of Vieques in Puerto Rico, USA, contains up to 700,000 tiny dinoflagellates per gallon (4.5 litres) of water. When agitated, these microscopic organisms (Pyrodinium bahamense, ‘whirling fire’) react by flashing a blue-green light for about one-tenth of a second. The island boasts other bioluminescent bays, but Mosquito Bay is the brightest – its narrow mouth prevents the dinoflagellates from being washed out to sea, and decaying mangrove swamps provide them with plentiful food.