Mosquito Bay

At night, an eerie ethereal light lights up Mosquito Bay, located on the south side of Puerto Rican island of Vieques. Bright blue tracer lines appear in the water as small fish dart through the blackness. But this illumination has nothing to do with spirits or aliens: Mosquito Bay happens to be one of the world's brightest bioluminescent bodies of water.

Bioluminescence is a form of natural light created by living organisms converting internal chemical energy into light. The light in Mosquito Bay is created by tiny organisms called dinoflagellates (specifically Pyrodinium bahamense or "swirling fire"). Mosquito Bay, also known as Bahía Bioluminiscente, contains an astonishing number—roughly 700,000 per gallon of water. Although they're microscopic, the effective size of the light they give off is a hundred times larger than their own bodies, and in great numbers they light up like an underwater aurora borealis.

Trapped by a curve in the bay's opening and fed by the surrounding mangroves, the dinoflagellates find Mosquito Bay a perfect home. The microscopic organisms make light upon sensing any movement in the water, swirling out bright blue-green plumes that eventually diffuse back into the dark stillness of the Bay. Their bioluminescence is thought to be a defense mechanism—the light attracts larger fish that can ward off the small fish who feed on dinoflagellates. It also makes paddling through the bay an astonishing and surreal experience.

When Hurricane Maria tore through Puerto Rico in 2017, Mosquito Bay was devastated. The storm's heavy wind and rain pushed water out of the bay and destroyed many of the mangroves that line its shores. The sudden, drastic changes in the environment were hard on the dinoflagellate population, and for a time the water went dark. Slowly but surely balance is being restored to the ecosystem of Mosquito Bay, and its signature blue glow has returned.

There are a number of outfits that will take people out to the "bio-bay," as it is called. Be sure to select one that uses kayaks not motor boats, and don't wear bug spray, as it can kill dinoflagellates. Visitors are asked not to swim in the water, as it threatens the delicate balance of the ecosystem. (Other bioluminescent bays have been severely damaged by visitors.) Try to visit on a night with no or little moonlight, as the bioluminescence is most impressive in near-total darkness.