The light comes from Pyrodinium bahamense, a species of dinoflagellate -- a single-celled organism classified as phytoplankton. When agitated by movement, each cell produces a flash of blue-green bioluminescence, a chemical reaction involving the enzyme luciferase. One cell's flash is invisible. But Puerto Mosquito contains staggering concentrations of them, and when billions fire simultaneously at the touch of a paddle or the passage of a fish, the effect is otherworldly.

Pyrodinium bahamense is found throughout the Caribbean, in bays across the Virgin Islands, Puerto Rico, and the Bahamas. But only a handful of locations produce bioluminescence visible year-round, and Puerto Mosquito is considered the brightest of them all. The bay's particular conditions -- its shallow depth, warm water temperature, surrounding mangrove forest that feeds the organisms with nutrients, and the near-total absence of modern development along its shores -- create an environment where the dinoflagellates thrive in concentrations found almost nowhere else.

According to local legend, the bay takes its name from the Mosquito, one of the ships belonging to Roberto Cofresi, the pirate whose exploits along Puerto Rico's coasts made him a folk hero in the early 19th century. Whether the story is true or simply a good tale attached to a memorable name, Puerto Mosquito has earned its own fame independent of pirate lore.

In 1980, the bay was proclaimed a National Natural Landmark by the United States government, recognizing its ecological significance. It is one of only three bioluminescent bays in Puerto Rico; the others are Laguna Grande in Fajardo and La Parguera in Lajas. Of the three, Puerto Mosquito consistently produces the most vivid displays. The surrounding mangrove forest and bay waters are now protected by the Vieques Bioluminescent Bay Natural Reserve, and swimming in the bay is prohibited to protect the delicate ecosystem. Visitors experience the bioluminescence from guided kayak tours that launch from the nearby beach community of Esperanza.

Puerto Mosquito's brilliance depends on darkness. The bioluminescence is most spectacular during the new moon, when no moonlight competes with the organisms' glow. On bright, moonlit nights, the display dims noticeably -- the dinoflagellates still flash, but the contrast vanishes. This relationship between darkness and light is central to the bay's fragile magic.

Light pollution poses the most significant long-term threat. The bay sits between the barrios of Puerto Ferro and Puerto Real on Vieques, an island whose small population and limited development have so far kept artificial light at bay. The U.S. Navy's decades-long occupation of much of Vieques, while deeply controversial, had the unintended effect of preventing coastal development along significant stretches of shoreline. Since the Navy's departure in 2003, conservation advocates have worked to ensure that development does not bring the streetlights, hotel signage, and beachfront illumination that would diminish what makes Puerto Mosquito extraordinary.

The kayak tours launch after dark from Esperanza, a small beach community on Vieques's southern coast. Guides lead groups through mangrove channels and into the open bay, where they instruct paddlers to drag their hands through the water and watch the light bloom. No motors are allowed. No swimming. The rules exist to protect both the dinoflagellates and the mangrove ecosystem that sustains them.

The experience is difficult to photograph -- cameras struggle with the particular quality of bioluminescent light, which is intense to the eye but diffuse to a sensor. This is, paradoxically, part of the bay's appeal in an age of documentation. You cannot Instagram Puerto Mosquito convincingly. You have to be there, in a kayak, on a dark night, watching your paddle paint blue fire across black water. The light exists only in the moment of disturbance, flaring and fading in less than a second. It cannot be bottled, replicated, or preserved. It can only be witnessed, an ancient chemical reaction performing its brief and silent show for anyone willing to sit in the dark and pay attention.