Tranquil Manialtepec Lagoon, a short 15-minute drive from Puerto Escondido, is home to an astonishing variety of bird and animal life and magnificent displays of tropical vegetation.

It's a popular spot for eco-excursions. The mangrove forests are an amazingly productive ecosystem; the roots nurturing the crabs, shrimps and mussels that attract a wide variety of fish species, that, in turn, draw the birds and other wildlife.

But when night falls on Manialtepec Lagoon and all the conditions are right, there are even more magical sights to be seen.

Manialtepec's waters are a veritable soup of tiny organisms: algae, protozoa, eggs, larvae and so on, known collectively as plankton.

One form of plankton gets particularly active as the waters heat up. It's called cyanobacteria, a single-celled algae containing chloroform and a blue substance which gives off an eerie phosphorescent glow when disturbed by currents.

At night, the thousands of fish darting through the waters leave streaks like falling stars, and the wake of a motor launch looks as though it has been illuminated by powerful floodlights. If you swim in the lagoon, the droplets fall from your skin like a shimmering shower of sparks.

It's a remarkable and highly recommended experience. Before planning an evening trip, check with the tour operators or the resort areas at the lagoon to determine that the phosphorescence is still happening; the phenomenon disappears as suddenly, and as mysteriously, as it begins.