Bahía Bioluminiscente (La Parguera), Lajas

The water at La Parguera looks ordinary by day. After dark, it's another thing entirely — dinoflagellates, millions of microscopic organisms, light up blue-green whenever the water moves around them. This is Bahía Fosforescente, one of three bioluminescent bays in Puerto Rico and one of seven year-round sites where that phenomenon is visible anywhere in the Caribbean. It is also, according to those who track these things, the most endangered of the three — human activity and mangrove loss have taken their toll. Go by boat tour, and go soon.