Laguna Grande

Laguna Grande is one of only a handful of consistently bioluminescent bays anywhere, and the mechanism is a single-celled organism, Pyrodinium bahamense, that flashes when the water around it is disturbed. Enclosed by mangroves that feed it and with a narrow mouth that keeps the population in, the lagoon lights up under a paddle blade in streaks of cold blue-green. Getting there means kayaking a mangrove channel roughly half a kilometre long in single file after dark, which is half the experience: the canopy closes overhead, crabs move in the roots, and the guides run the trip on headlamps and whistles. It is the easiest of the three bays to reach from San Juan.

What you actually see

• The glow - blue-green light in the water under every paddle stroke.
• The mangrove channel - a narrow tunnel of roots paddled in the dark.
• Las Cabezas de San Juan - the reserve and lighthouse above the lagoon.
• The night sky - dark enough here for stars over the water.

When to go

Go on or near a new moon: moonlight washes the effect out almost completely, and reputable operators say so. December to April is drier with calmer conditions. Rain and wind cancel trips. Book ahead, as the number of boats permitted each night is limited.

Honest expectations

The glow is subtle, and photographs online are almost universally enhanced beyond what the eye sees; go for the experience rather than the picture. Sunscreen and insect repellent are banned in the water because they harm the organisms. Mosquito Bay on Vieques is significantly brighter, and anyone able to get there should. Trips are cancelled often.

Nearby

Seven Seas beach is nearby; Culebra and Vieques ferries leave from Ceiba.