Locals claim that Bahía Mosquito, a designated wildlife preserve, has the highest concentration of phosphorescent dynoflagellates not only in Puerto Rico, but in the world. Indeed, it's also known as Bioluminescent Bay - and it's magnificent. A trip through the lagoon is nothing short of psychedelic, with hundreds of fish whipping up bright-green sparkles below the surface as your kayak or electric boat passes by, but the best part is when you stop to swim: it's like bathing in the stars.

Don't ever accept a ride in a motorized boat - the engine pollution kills the organisms that create phosphorescence. You can just drive east on the rough Sun Bay road (you'd better have a 4WD because the road's a mess) and jump in for a swim at any point that's glowing. However, an organized trip will give you far more opportunity to really take in the spread of phosphorescence. There's another inlet to the east, Barracuda Bay, that's also filled with dynoflagellates, but most tour operators don't venture out that far. Reservations are recommended for tours and kayak rentals in high season.