Imagine paddling a kayak across a dark bay on a moonless night. The stars are out, the air is warm, and the water looks black. Then you dip your paddle in, and the water explodes with light.

Blue-green sparks swirl around the blade. Drops falling from the paddle glow like tiny stars. Fish dart beneath the kayak, leaving trails of light. If you trail your hand in the water, it lights up as though you are touching liquid fire.

This is Mosquito Bay, on the island of Vieques, in Puerto Rico. It is one of the brightest bioluminescent bays in the world.

Living light

Bioluminescence is light produced by living things. Fireflies use it, as do many deep-sea fish, jellyfish, fungi and worms. In the ocean, it is extremely common. Some scientists estimate that most animals living in the open ocean's darker depths can produce light.

In Mosquito Bay, the light comes from tiny single-celled organisms called dinoflagellates. The main species is Pyrodinium bahamense, which means roughly "whirling fire of the Bahamas". Each cell is far too small to see with the naked eye.

How it works

The dinoflagellates produce light using a chemical reaction. A molecule called luciferin reacts with oxygen, helped by an enzyme called luciferase, and releases energy as light. The word luciferin comes from the Latin for "light-bringing".

The cells don't glow all the time. They flash when the water around them is disturbed: when a wave breaks, a fish swims past, or a paddle moves through. Pressure on the cell triggers a rapid chemical change, and it emits a brief flash, lasting less than a second.

Why this bay?

Dinoflagellates live in oceans all over the world. Sometimes, they create glowing waves on beaches. But in most places, the light is faint and patchy. What makes Mosquito Bay so special is concentration. Researchers have counted very large numbers of dinoflagellates per litre of water.

Several conditions help. The bay is shallow and warm. Its entrance to the sea is narrow, so the water doesn't flush out quickly and the organisms stay inside. It is surrounded by red mangrove trees, whose fallen leaves and roots release nutrients, possibly including vitamin B12, which the dinoflagellates need.

A fragile glow

The glow depends on a delicate balance. Pollution, lights from development and changes to the mangroves can reduce it. Sunscreen and insect repellent can harm the organisms, which is one reason swimming in Mosquito Bay has been banned. Visitors must explore by kayak or electric boat.

In September 2017, Hurricane Maria struck Puerto Rico with devastating force. The storm churned up the bay and damaged mangroves. For months afterward, the glow was dramatically dimmer. Then, gradually, it returned. Scientists and local guides celebrated as the bay began to glow brightly again.

Glowing elsewhere

Bioluminescent displays happen in other places around the world. In the Maldives, beaches sometimes sparkle with glowing plankton. In Jervis Bay, Australia, and Toyama Bay, Japan, there are glowing displays at certain times of year. California's coast sometimes experiences glowing blue waves during algae blooms.

But few places glow as reliably as Puerto Rico's bays, which is why people travel from all over the world to see them.

Seeing it

The best time to visit is on a dark night, ideally around the new moon, when moonlight doesn't drown out the glow. Photographs struggle to capture it; the flashes are too brief and faint for most cameras. Many visitors say it's one of those experiences that can only be understood in person.