Mosquito Bay, close to Esperanza, is one of the world's brightest bioluminescent bays, and when you dive into the warm water after dark, your body will glow white. That's because the mangrove-surrounded inlet is perfectly sheltered and contains a very high concentration of dinoflagellites, small incandescent microorganisms that set the entire bay aglow at night. Lots of tour operators take boats out into the bay—the small inlet can get pretty packed come sunset—but two operators get our vote. The environmentally conscious Island Adventures ((787)741-0720) takes guests out on electric pontoon boats (which don't pollute the bay) and provides a fascinating lecture on the biology behind the natural phenomenon. Kayak tours with Aqua Frenzy ((787)741-0913 or (787)240-7985) are also top-notch; owner Tim Raymond has more than 11 years of experience working in Mosquito Bay and has developed an extended tour where kayakers explore the island's mangrove forests, go bird spotting, and then enjoy a waterside meal on a beach accessible only by boat. The tour ends with time in the bay after dark.