When I was in my early 20s I traveled to Puerto Rico on vacation with some friends from high school. We sat on the beach and drank fruity drinks with tiny umbrellas, visited the colonial fort in old San Juan (a place that, with its rolling green meadows and stone turrets perched just above the ocean cliffs, looked to me like Narnia) and for several days we stayed in a rental in Vieques.

The diminutive island eight miles east of the mainland was for many years a U.S. naval base. Much of the heavily forested island was made into a wildlife preserve, which is now off-limits. But the rest of the island has retained a similar kind of rural, unspoiled beauty. There are white sand beaches and coral reefs, and even feral horses that trot around the pastel-colored houses. But Vieques’ most remarkable natural feature is its Bioluminescent Bay.

I went to the Bio Bay at night, on a bus that departed from the tiny town of Esperanza and wound its way east along the coast. It was perfectly dark when we arrived, and silent, except for the sound of insects and giggling tourists. Our tour guides produced canoes, and we filed in by twos and threes, paddling out to the center of the bay.

The water was black and glassy, but at the appointed time we jumped in to meet the creatures that give the Bio Bay its name. As we landed in the murk with one splash after another, the water around us flashed with a bright, milky blue glow, illuminating our limbs and reflecting up onto our faces. I swept my arm through the water and watched as it left a trail of blue stardust lit up behind it.

The Bio Bay, you see, is home to millions upon millions of tiny, one-celled microorganisms called dinoflagellates – in this case tiny marine plankton that are among the earth’s many bioluminescent creatures. They produce their eerie light when they’re disturbed, as they were when we decided to take a midnight swim in their home.

“What’s the point of that light?” J. Woodland “Woody” Hastings asked at a recent Chicago lecture. The Harvard professor of Natural Sciences studies bioluminescence in creatures across the spectrum of life, from simple, one-celled bacteria to angler fish that swim in the deepest, darkest depths of the ocean and carry their light around with them.

Hastings said this is the question he’s invariably asked at his talks. In the case of one such organism he’s studied, a luminous mushroom found in the Brazilian rain forest, Hastings posited that the glow of the fungi attracts insects, which will eat the mushroom and help disperse its spores. But in the case of the plankton in the Bio Bay, my tour guide had another explanation: supposedly, he said, the glow was meant to act like a “burglar alarm,” meant to attract a secondary predator that would threaten and scare away the primary predator bothering the dinoflagellates.

As my tour guide spoke, I felt a blindingly painful sting on my left calf. A jellyfish that I could not see – but which had clearly seen me – had wrapped its tentacle around my leg. I hauled myself out of the water and back into the boat, howling with pain. Nature at work!

In the audio above you can hear Hastings’ account of another mystical spot of bioluminescent water, this time in the Indian Ocean, known to generations of sailors as the “milky sea.” And, you can hear more about the spectrum of creatures that cause our waters to glow like a softly lit siren.

Dynamic Range showcases hidden gems unearthed from Chicago Amplified’s vast archive of public events and appears on weekends. Woody Hastings spoke at an event presented by the Chicago Council on Science and Technology in February of 2013. Click here to hear the event in its entirety.

Robin Amer is a producer on WBEZ’s digital team. Follow her on Twitter @rsamer.