I’m standing in a narrow alleyway when a stranger approaches to tell me that he can channel the power of the ocean. Crazy? Maybe. But I’m on the island of Vieques with a similarly far-fetched quest: to swim in a celestial sea. I tell the man, who introduces himself as Charlie the Wavemaster, that the Milky Way will soon crackle and shimmer as it slips through my fingers. Bits of stardust will cling to my hair.

Vieques’s Mosquito Bay, also called Bioluminescent Bay or Bio Bay, is one of the last ecosystems in the world where dinoflagellates — microscopic, single-celled organisms — create halos of light around whatever disturbs their nightly flotation. Marine bioluminescence, which appears to mirror stars in the night sky, occurs spontaneously around the globe, but no site on Earth hosts the phenomenon with more regularity than the southern coast of Vieques. Guinness World Records 2008 named Bio Bay the brightest in the world.

“The Bio Bay, it’s all about vibrations,” Charlie says. “You slap the water, and it lights up! It’s inspiring! The water holds so much awe!”

Charlie is wearing a baseball cap and handkerchief headband, and he’s holding a long metal pole, which I gesture toward. “Oh, this is my magic wand!” he says. Charlie taps it on the ground near my feet. “I’m putting out vibrations right now. Feel it?” There is a dull resonation under my sand-encrusted flip-flops.

When I turn to continue my exploration of the town of Isabel Segunda, Charlie follows. Cars blast the thump-de-thump of reggaeton. Neighbors chat through barred windows with people on the street. Young men in athletic clothing ride bareback on horses guided by rough, twisted rope. Roosters run wild through the scene, necks pumping back and forth as they dodge tires and hooves.

Just before Charlie bids me farewell, slipping into the driver’s seat of a borrowed pickup, he says: “There are so many mysterious ways and miracles in the world. There’s so much involved, you could never understand it all.” As he drives away, I can hear his metal rod echoing in the truck bed like a tuning fork.

* * *

Vieques, which is about eight miles from the main island of Puerto Rico and has a total population of roughly 9,000, is a place where it hasn’t always been easy for residents to see the bright side of things. In the 1940s, thousands of people were forced from their homes when the U.S. Navy expropriated roughly two-thirds of the 21-mile-long, four-mile-wide island for artillery storage and military training. In the following decades, Vieques was the site of perpetual military training involving munitions that delivered doses of napalm, lead, depleted uranium and a cocktail of other contaminants. In 2003, when the Navy ceased bombing, nearly 18,000 acres were designated as a national wildlife refuge. This move has kept construction concentrated in a narrow swath of land in the center of the island, preserving its status as one of the least developed in the Caribbean.

In 2005, the Environmental Protection Agency added Vieques to its list of Superfund National Priorities, officially designating parts of the island hazardous waste sites. This designation made Vieques a supremely unlikely travel destination, but the Navy’s toxic legacy has proved no match for 50 undeveloped beaches where — on a busy day — visitors might share a crescent of sand with one or two other intrepid souls.

Garry Lowe — a lanky guy who’s almost always accompanied by his dog, Lola — counts on Vieques’s growing tourism industry for his livelihood. Not long ago, Lowe was a fly-fishing guide in Colorado. On a whim, he decided to return to Puerto Rico — his mother’s homeland and the site of good boyhood memories — to start Vieques Adventure Co.

“I really shook up my life when I came down here,” he says. “It was like Boggle!” He hops around, pretending he has a handful of dice and then points to the ground, to indicate he has just rolled a series of letters in his imaginary board game. He shouts, “L-I-F-E! This is my life! I won! I’m on the water 200 days a year, and work isn’t like work at all!”

Lowe runs biking and fly-fishing tours, but his real claim to Vieques fame lies in his innovative use of clear canoes. His polycarbonate boats give tour group members who are unwilling to swim in Bio Bay at night an idea of what it might be like to be immersed in its unique aquatic wonders.

Tonight’s group gathers in a puddle of yellow created by the headlights of Lowe’s passenger van. He suggests a last-minute run into a nearby market to use the facilities.

When the crowd — a mix of all ages — emerges from the store, Lowe hangs his head as if in mourning. He has spotted a teenage boy in too-tight swim trunks toting a new bottle of bug spray. “I asked you not to use any of that stuff!” Lowe says. He points to the store. “Go in and wash your arms. Scrub them!”

“Really?” the boy asks.

“Yes!” Lowe says. “I know you don’t want to kill anything on purpose!” He shakes his head. This is the sort of thing he fears most. “I’m not a tree-hugging kind of guy, but I do think that humans have a habit of wanting to save something after it’s already gone. Everything we put on our bodies has the potential to kill the dinoflagellates. Shampoo, deodorant, sunscreen. All of it.”

A substantial loss of the photosynthesizing plankton would disrupt the Bio Bay’s food chain and result in decreased oxygen levels. It would also snuff out conservationists’ hope that — through careful regulation and vigilant guiding — tourists can safely commune with Earth’s remaining bioluminescent bays. There are about six left in the world, and Puerto Rico is home to three.

Lowe’s watch beeps. Usually, island time means “hang loose, no worries” and, in daylight hours, this is true on Vieques. But at night, the demand for tours, especially Lowe’s, is too great to wait for stragglers. “We’re still missing one. I feel bad she’s going to miss this,” Lowe says. “We’ll be there in four minutes and 48 seconds!”

When we reach the put-in site, Lowe puts on a headlamp to show us around. His boats are all outfitted with red lights so that he can keep track of his crew. The color was chosen for its benign nature: White light has the potential to disrupt the plankton’s circadian rhythm, the biological clock that lets them know when it’s time to glow.

Once Lowe has safely positioned everyone in canoes, he hops into his own, opaque kayak. Almost immediately, the wake alongside our boats is illuminated. I feel as if I’m paddling through neon blue champagne bubbles. I put my hand in the water. When it’s immersed, I can see my fingers outlined in tiny orbs of light. When I raise my hand to the sky, it is camouflaged by spontaneous twinkling.

Lowe has located a buoy and invited the group to gather around him. He explains that the dinoflagellates are neither plant nor animal, but rather, as the Latin origins of the word bioluminescence indicate, they are living light. “They use the same chemicals as lightning bugs,” Lowe says. “They’re both trying to talk with light, just in different ways.”

A rope connecting our boats hits the water in a metered motion, setting off liquid sparks. The group is abuzz. Someone asks, “Would the water still glow if I took some in a jar and put it on a shelf in my house?”

“It wouldn’t last,” Lowe says. “It would just be water in a glass if you took it away from this place. ... It’s just like with people: You’re going to thrive if you’re where you’re supposed to be.”

He straddles his boat and recounts a childhood trip to La Parguera, a bioluminescent bay on mainland Puerto Rico that has nearly lost its luminosity because of pollution and sedimentation. “La Parguera has been dead for a long time. Even when I was a kid, the closest you could get to the water there was when they poured it bucket-to-bucket to show you the glow,” Lowe says. “At some point, it might not be possible to touch the water here anymore.” The group is quiet as we turn our boats toward land.

We paddle close to shore. A woman, who has revealed that she’s staying at the only gated resort on the island, says softly: “He’s right, you know. It’s amazing that we’re allowed in here at all.”

* * *

The next morning, at Lowe’s suggestion, I walk along the picturesque main board-walk of Esperanza — the town closest to the bay — until I reach the Vieques Conservation & Historical Trust, an unassuming building set back from the road. I make my way through a museum of indigenous Taino artifacts before entering an educational center maintained by the trust’s director of community affairs, Mark Martin Bras. He emerges from a side door holding a giant stuffed squid.

Bras’s duties include educational programming and acting as liaison for the trust’s research partnerships with universities, but he moonlights as a guide for Island Adventures, one of the most popular tour companies on the island. “I’ve seen people get religious about the bay,” he says, “I’ve seen people cry. Adults start to talk like children. They’re not losing intelligence; they’re just disarmed. Nature has the power to completely disarm people. Most of the time we live outside of that power, but the bay returns it to us.”

Near dusk, I join Bras on one of Island Adventures’ repurposed school buses. We’re on our way to the bay’s main entrance, inside the public beach compound of Sun Bay. The sides of the bus screech out as branches claw at its paint. When the vehicle reaches its destination, a clearing roughly 20 feet across, a woman in front of me says: “Those would have been great sound effects for a horror film! Actually, this whole trip would be a great plot for a movie.” She changes her tone to that of a mock announcer, “They went to see the dinoflagellates, and they never came back!”

We leave the bus and stumble to find our footing as we’re guided, in single file, onto narrow metal boards leading us onto an electric pontoon boat. On board, people seem desperate to photograph the bay, though the bioluminescence is not yet visible. Bras asks guests to turn off their flashes and warns, “There is a gap in technology and the bay.”

There are few photographers who have been able to document the bay in a meaningful way, and rarely do they achieve good results without digital alterations. In a world of Google maps — when it’s possible to virtually fly over the Eiffel Tower and the Grand Canyon on your lunch hour — the bay remains a you-had-to-be-there experience.

Bras raises his arm to gesture toward a hilltop neighborhood visible from the water, saying, “Light pollution is one of our greatest enemies here.”

One tourist — who is still struggling to photograph the bay despite Bras’s comments on its elusiveness — calls him over to ask if he wouldn’t mind turning up the boat’s lights a little. She doesn’t yet understand what she’s looking for, but she doesn’t want to miss documenting it when it appears.

Bras says: “That’s not coming from the boat. That’s the bioluminescence you’re seeing out there.”

The woman, still struggling with the concept, says, “There’s no light?”

“No artificial light,” Bras says.

From the boat’s perspective, high above the water, the bioluminescence reveals the back-and-forth patterns of fish swimming, squiggly lines drawn in their wake.

Bras points toward the surrounding mangroves and says, “Mangrove trees actually create land.” He directs our attention to the silhouette of a hillside and says: “See that? It used to be water all the way up to the mountain.” A level mangrove forest, creeping into the bay’s placid waters, is just visible in the evening’s fading light. Mangroves, which shed nutrient-rich leaves, help maintain the bay’s heightened salinity and build islands as they strain sedimentation.

Someone asks of the bay’s narrow, ocean-gulping mouth, “Will the mangroves ever close the opening?”

“Well, they could, but humans would never let that happen,” Bras says.

He bellows: “There is a story. I don’t want anybody to think I agree or believe in the story, but it goes like this: When they came here, the Spanish thought this was the devil’s water, so they put rocks around the mouth to keep the devil in.”

Two chemicals that produce the plankton’s light — luciferin and luciferase — were named after Lucifer, the fallen angel. “Historically, most people that were writing things down were thrown toward evil more than good,” he says. “They had the idea that bioluminescence was too surreal to be natural.”

Bras gives a small sigh and continues: “I know people have a desire for mythology — and it has its own beauty — but I think that story takes away from the mangrove and nature. It gives humans the credit for creating this.”

Our slow-moving pontoon comes to a stop. It’s time to swim. We quietly stargaze as we wait our turn to use the boat’s ladder. When a cloud threatens to cover a section of sky to our left, Bras pulls a laser pointer from his pocket and shoots it into the heavens: “Before it goes away, there’s the king of the gods: Jupiter! And there’s Orion’s torso!” He traces the body — celestial belt, outstretched sword — then, without pause, he abandons his set course to trail a shooting star with his red light saber.

“Do that again!” someone calls out, as if Bras might be able to manipulate the sky above.

I approach the edge of the deck to see swimmers kick their legs and flail their arms. They appear as iridescent butterflies shimmering in an inky sky. I clip a slender piece of foam around my waist and probe the darkness. The water is warmer than the air. It feels near body temperature, and, when I am fully submerged, I am surrounded by a cloud of liquid electricity. I am speaking to the water through movement, sign language, and it is responding with visual cues.

Not far from where I’m treading in a tornado of light, a man is carrying on as if he’s on hallucinogens. “Look at the hairs on my arm! Look! The hairs on my arm!” He dips his arm into the bay and lifts it for everyone to see. His hair has become a catchment of flickering pulsars. “It’s so weird! Like I’m covered in stars!”

When Bras announces that it’s time to get out of the water, a collective groan rises. Back on shore, when people begin to make their way down the boat’s gangplanks, Bras shouts after them: “Don’t forget to tell people about the lights!” He isn’t talking about the soul-stirring bioluminescence; he’s referring to the street lamps and front porch bulbs in the neighborhood above the bay. He’s reminding us to take inventory of our far-off corners of the world, where light pollution is likely veiling our view of the night sky.

“More light,” Bras says, “isn’t always better.”

* * *

During my last day on Vieques, I decide to take a daylight tour of the mangroves. I’m part of a large crew, but we’re attentive as Carlos Cruz Morales, a 20-something with wavy black hair, gives a brief paddling lesson.

The group falls into loose formation as we travel toward a mass of mangroves. It’s unclear how we’re going to pass through the forest’s tight aerial root system. I am within a foot of the mangrove forest, and, still, I do not see a clearing large enough for a human body, much less a boat, to pass through.

Morales hops out of his kayak. The water barely reaches his chest. “Abracadabra, Rastaman! Open, sesame!” he shouts dramatically as he moves a few of the trees’ dreadlock-like, dangling prop roots to reveal a passage.

He pushes the first craft through. The tunnel seems to close itself behind the kayak. The mangroves might not be home to a plethora of spiders, as one fellow boater had feared, but they host a healthy population of tarantula-size arbor crabs, which scamper along the roots as I pull my paddle into my boat.

This mangrove forest is relatively young, with most trees standing at 25 to 30 feet. During Hurricane Hugo, it lost many of its mature trees, some of which were more than 60 feet tall. “After Hugo, the bay didn’t glow for six months,” says Morales’s colleague, Michelle McNerney. “The conditions just weren’t quite right.”

We’ve traveled half a mile into the mangrove when we reach another open section, a watery roundabout. Clumps of spongy silt float up around us. Morales jumps out and thrusts his hands into the water to produce a huge clump of earth, which he plops on top of his head as the group looks on, aghast. He begins to work it into his long hair. “What’s good for the dinoflagellates is good for your skin and nails!” He continues his mud bath, clearly delighted by the group’s collective recoil. Each time he pulls a handful of mud from the water, a sulphuric stench wafts over our flotilla.

Morales looks slightly alarmed when a military-looking cargo plane flies by overhead. “I haven’t seen an airplane like that in a long time,” he says. Morales grew up with the military presence. “I used to sleep every night to the sound of bombs. Boom! Boom! It thundered every day without rain.”

When the plane is gone, Morales dips back into the swampland. Still reminiscing, he explains that one of his favorite childhood haunts was a beach where purple and orange coral formed rainbow cliffs. To his chagrin, it was located in a former military zone. “Sometimes, when they were bombing, I wouldn’t go there,” he says. “Other times, they still said I couldn’t go there. I did, anyway. But then the protests began.”

People opposing Navy activities on the island — including well-known activists such as the Rev. Al Sharpton and Robert F. Kennedy Jr. — were regularly arrested for acts of civil disobedience between 1999 and 2003. The movement was spurred when a local man was killed by an off-course bomb, an incident that incited international rage. “When things got bad, I couldn’t go to my beach at all,” Morales says. “I would have gone to jail if they’d found me there.”

Morales jumps in his boat to lead us back through the mangrove. As he paddles, he mentions that some of his friends are advocating a local movement to build a bridge to mainland Puerto Rico. “People always say, ‘I hope Vieques gets a Kmart’ or something like that, but I say take $2 and go to the mainland on the ferry. It’s all there.”

He negotiates the mangrove passageway gingerly, using his hands to propel himself root-by-root slowly, so that his boat doesn’t hit the oysters below. He says, “I know Vieques will change, but I want it to stay like this — quiet,” he says. “A bridge would make the island different. If we have more people, we need bigger roads. Bigger roads mean bigger neighborhoods and more cars. Nature will disappear.”

A year ago, Morales had a different outlook, he says, “I was like, ‘Nature, who cares about that?’ ” But when he heard Abe’s Snorkeling was hiring, he was quick to apply. Though he couldn’t yet speak English, he knew how to kayak because he had grown up around boats, he had learned to snorkel as a spearfisherman, and he was familiar with most parts of the island because it was his home. Even so, when Morales was hired, he didn’t see any fringe benefits to working outdoors.

Over time, his feelings began to change. He says: “Before, I didn’t understand that you don’t need money for the most beautiful things. A smile? Free! The beach? Free! Surf? Free! Free things for me are enough. That’s what people come here for, what they really need, the free things. My island isn’t developed, and that is a good thing.”

When we exit the forest, he says: “When I started working here, I didn’t know how these wonderful trees worked. Everything here is important.” He shakes his head, “All my life changed when I starting sharing my island. I’m no santo, how do you say, saint? But I’m a better person since I got to understand this place.”

We’re in full sunlight now, back in the open waters of the bay. The water becomes choppy as we round cliffs on our way to a small beach. I guide my kayak onto coral-littered sand, and a middle-age man, who’s on Vieques as part of a trip around the world with his wife, pulls up beside me and plants his paddle as a flagpole.

The man says, “It’s unbelievable this place is still here.” But it’s really what isn’t on Vieques that’s amazing. Even now, nearly a decade after the Navy stopped bombing the island, there are no chain stores. No fast-food restaurants. No golf courses. “It’s like the whole island is 50 years behind,” he says. He pauses, glances around the otherwise deserted beach, and reconsiders. “Or maybe it’s 50 years ahead.”

Leigh Ann Henion’s debut book, about wonder-inducing natural phenomena, is forthcoming from the Penguin Press. She can be reached at wpmagazine@washpost.com.