Lying just inside the reserve, the placid waters of Laguna Grande look fairly ordinary by day, but when night falls everything changes. Thanks to creatures known as dinoflagellates, kayaks and boats leave glowing trails in the dark, while water falls like sparks of light from paddles and trailing arms. Puerto Rico has several places where heavy concentrations of microscopic plankton create this mesmerizing phenomenon: Vieques is home to the most celebrated example, but on a dark (and moonless) night, Laguna Grande is almost as magical. Optimum days for viewing are based not only on the phases of the moon but also the actual time the moon rises – check before you go.

The only way to experience the lagoon is to take a tour, preferably by kayak. It is forbidden to swim in the bay, so cutting through the mangroves by kayak is the best way to appreciate its bizarre luminescence – it’s not as taxing as it looks and easy for beginners. One of the most eco-friendly and informative operators is Yokahú Kayak Trips (787/604-7375, http://www.yokahukayaks.com), which runs 2hr tours at 6pm and 8pm daily. Kayaking Puerto Rico (787/435-1665, http://www.kayakingpuertorico.com) is another professional outfit that can arrange trips also for $45 per person. Try to book at least three days in advance for both companies.

Captain Charlie Robles’s electric boat at Bio Island (787/422-7857, http://www.bioislandpr.com) is an eco-friendly alternative to kayaking. Captain Suárez (787/655-2739 or 787/556-8291) is the only operator licensed to pilot actual motorboats in the lagoon, and though he’s a knowledgeable guide, his boat isn’t really helping the lagoon. All tours start at the Las Croabas quay, last 90 minutes, and guides are all bilingual.