The first glimpse of Laguna Bacalar felt like a mirage, a flash of luminous turquoise through a haze of trees. Twenty-six miles long and just over a mile wide, the “Lake of Seven Colors” snakes through the jungle, carrying tales of Mayan origins and pirate attacks.

The shifting hues, courtesy of the lagoon’s white, limestone bottom, practically demand to be photographed and I gave in, but my photo was no comparison to the real thing, or to the 443,000 and counting #bacalar images on Instagram, showing the lagoon from different vantage points, at different times of day, with beautiful people on waterfront swings and boatloads of revelers raising cold beers.

If you look closely, those photos show other things as well: Colors that, while stunning, are already diminished. People standing and walking on stromatolites, ancient life forms of great scientific significance. Pollution, literal black clouds in azure waters, ignored or unnoticed by those happy couples and families and friends.

I’d started hearing whispers about the lake and the town of Bacalar in the far south of Quintana Roo early last year: from an Uber driver in Denver, outside a mezcal bar in Mexico City, from a divemaster up the coast in Cozumel.

I’d come to see it for myself in May, and returned in November; it felt as miraculous as the first time. But my second visit also amplified my questions about Bacalar. Was the lagoon’s otherworldly beauty, its utter Instagrammability, going to be its downfall? Would these images, snapshots of carefree holidays, become poignant reminders of something lost? And was my presence only going to hasten the destruction?

The “Wild West”

Bacalar’s name is thought to come from the Mayan “Bakhalal,” meaning place of reeds. Colonized by the Spanish in the 16th century, it sits just uphill from the lagoon and is centered around a large main square, or zócalo, anchored by the Fort of San Felipe. Sleepy in the humid jungle heat, the town remains compact and walkable; a coast road runs along the lagoon’s southern shore and is easily accessible by car or bicycle.

Fernando Gaza, the owner of Casa Tortuga, came to Bacalar in 2013 from Monterrey, Mexico. The waterfront hostel, which opened that same year, was one of the first on the shores of the lagoon.

“It was this hidden jewel. When you drive from Chetumal to Tulum, or Playa del Carmen, you don’t even see it,” he said. “Most people had just heard about it through word of mouth. But even then we would say, ‘How many years until this is like Tulum?’”

Comparisons to Tulum, 133 miles north and the latest example of explosive growth on Quintana Roo’s Costa Maya, are probably inevitable. Like that town, Bacalar has cenotes, natural swimming holes in the limestone rock; early Mayan archaeological sites (Dzibanche and Kohunlich); and, of course, that gorgeous, photogenic water. But Tulum is now also synonymous with thoughtless overdevelopment. Before Tulum, it was Playa del Carmen; before Playa del Carmen, Cancún.

Rapid growth has already hit Bacalar — Mr. Garza noticed a major increase in development, and foreign tourists, about three years ago, with new businesses springing up in the last six months.

Some of this change is a boon for travelers. Accommodations in Bacalar range from backpacker-friendly hostels — at Casa Tortuga, beds in shared dormitories are $20 per night, and a private room with a bathroom is $80 — to luxury resorts with rooms starting at $400 a night. Hotels and hostels on the waterfront, by and large, are pricier than those in town.

Options for food and drink abound, from local, wood-fired fare and a mainly Mexican wine list at Nixtamal to a breakfast of eggs doused with fiery peanut-and-pumpkin-seed salsa at Enamora. One evening, after a round of excellent al pastor and Cochinita Pibil tacos at Mestizos, I doubled down at Barbanegra, a buzzy taqueria and cantina with a steampunk-pirate beach vibe, filled to capacity. Their fried cauliflower tacos, layered with carrot, cabbage and guacamole were as delicious as I remembered.

Dinner at Nixtamal, one of the higher-end options in town, might start around $20 per person, while a hearty spread of tacos and a beer at Mestizos will be closer to to $7 (street food carts around the zócalo are even more affordable).

But an even bigger boom is on the horizon courtesy of the Mayan Train, a 950-mile rail line meant to stretch through the states of Quintana Roo, Tabasco, Campeche, Chiapas and Yucatán, with a dedicated stop in Bacalar, slated to be completed in 2023. With the arrival of the train, Bacalar’s relative inaccessibility — it’s a four-hour drive from the Cancún airport — will no longer be a hindrance to even more explosive growth.

“Over the last two years, the market has appreciated 350 percent,” said Ryan Gravel, the owner of the real estate brokerage Buy Bacalar. “Bacalar is new, it’s groovy. It’s the Wild West.” He paused. “But the ecosystem here — it’s so fragile. We could easily destroy it.”

Environmental Implications

Cenote Cocalitos was quiet when I arrived at 11 a.m.; by noon, the sun emerged and the crowd grew steadily, with visitors piling their belongings under a rickety palapa and gingerly wading into the water down algae-slicked wooden steps. A Dutch couple swam to a set of half-submerged hammocks. The scene got a sudden injection of energy when three Mexican teenagers arrived with a cellphone blasting banda music sheathed in a waterproof case.

Some, but not all of the visitors, stopped to read the signs explaining the structure of the cenote, and highlighting the presence of stromatolites, which, according to the Bacalar-based independent biologist Shanty Acosta Sinencio, were the first photosynthetic life forms on earth. Formed by thousands of layers of calcium carbonate-secreting cyanobacteria, stromatolites resemble giant boulders but are, in fact, living beings. Bacalar’s oldest stromatolites are between 7,000 and 10,000 years old.

“These organisms grow in very particular environments and have been found in very few places in the world,” Ms. Acosta said. According to the biologist Silvana Ibarra Madrigal, who works with the Mexican government as an environmental assessor, Bacalar is home to the largest known group of freshwater stromatolites in the world.

The stromatolites, and the lagoon itself, are visibly affected by Bacalar’s explosive growth, according to Ms. Acosta and Ms. Ibarra. Much of the issue is structural — the town’s waste treatment facilities are outdated and overcapacity, causing sewage to flow into the lagoon, particularly during hurricane season (the vast majority of households aren’t connected to the system at all). Garbage collection can be spotty, and separation is nonexistent. But tourists can do damage, too — at the cenote, I saw people slathering on sunscreen before entering the water and clambering on the rocklike structures as they angled for better photos.

Part of the problem, Ms. Acosta said, is that environmental care isn’t prioritized by the local government or among some members of the local population. She said that the lack of long-term planning remains a major hindrance to making meaningful, widespread change, and that organized crime has also contributed to socio-economic deterioration.

Kamila Chomicz, an artist and biologist from Gdansk, Poland, who has worked closely with Bacalar’s activist community, said there is often resistance about making these concerns public, lest tourists be deterred from visiting.

Cocalitos, in addition to the signs imploring visitors to not walk on the stromatolites, has roped off some areas to help swimmers avoid contact with them. But the barriers, and information, can be easy to miss, or to ignore.

A Different Kind of Tourism

A growing number of local business owners are trying to inform tourists of the area’s fragility. Rooms at Casa Tortuga and Casa Chukum, a sleek boutique hotel that opened north of the zócalo last spring, have printed materials in their rooms with information about the stromatolites and the best way to interact with the lagoon.

Many of these materials are thanks to Marilina Labat, a local designer who volunteered her services to Dirección Ecología Bacalar, a government organization focused on raising ecological awareness. As of September, the group had set up a large display dedicated to the stromatolites in Spanish and English in the town zócalo. Nearby, at El Manati, a spacious cafe, gallery and shop that offers assistance in booking eco-conscious experiences, information about the lagoon is displayed alongside exuberant artistic renderings of stromatolites. Ms. Chomicz thinks that there’s a major opportunity to bridge the gap between science, the local community and tourists — she put together an exhibition of photos and videos focused on the stromatolites and lagoon last spring, and has since made a short documentary on the same topic.

The mission of Bacalart, a free art festival founded by Catalina Sour Vasquez, a French artist who has made numerous trips to Bacalar, and held for the first time this November, is similar. A collaboration between META, Ms. Sour Vasquez’s Paris-based artist’s collective and a number of Mexican artists, Bacalart combined art exhibitions, musical performances and film screenings (including films from Ms. Chomicz) at three venues around town — Hotel Makaabá, a luxury boutique hotel that opened last summer; Damajuana, a Tulum-based sushi restaurant and mezcaleria on the waterfront and El Galeón Pirata, a cultural center and music venue.

“Maybe, in addition to raising awareness of the lagoon, we can help bring a different kind of tourist here — people who are interested in art and culture, not just partying,” said the artist and META member Diane Merli.

It’s not a far stretch. In addition to permanent gallery spaces like El Manati, walls around town are covered with intricate street art depicting aquatic life and environmental appeals. But is art, and heightened awareness, enough to influence the direction of Bacalar’s development?

The Responsible Tourist

These questions weighed on me as I boarded Yemaya, a 26-foot sailboat, for an afternoon trip around the lagoon with Eco Sailing Bacalar, booked through El Manati (sailboats, paddleboards and kayaks are greatly preferable to motorboats, I was told; engines, if used, should be four-stroke). We set off for the Pirate Canal, a waterway that let raiders sneak into the lagoon to sack the town in the 17th century. Boats, some running massive, two-stroke engines, gathered around a skeleton of concrete and rebar covered in graffiti — a failed restaurant development turned photo-op.

It’s easy to make development the catchall culprit here. But done thoughtfully, and with transparency, it can be beneficial. At Hotel Makaabá, an open, concrete grid rises above a multilevel, sun-warmed pool meant to mimic the lagoon and its cenotes. The proprietor, Allen Patiño, originally from Mexico City, said that part of the appeal to guests is the peace of mind of leaving as minimal a footprint as possible. Everything, from the waste treatment system designed by the Brazilian firm Lowbio to the water-filtering plants in rooftop rain cisterns, is meant to create a self-sustaining system, all of which is explained in detail on the hotel’s website.

“The hotel is a tool to change people’s minds,” Mr. Patiño explains. “There are viable solutions to the problems we are facing. It’s possible to balance caring about nature and doing good business.”

Samantha Calzada Valencia and her husband, Bernardo Buendía Bosch, hope to have a similar impact with Azul Nomeolvides, their six-cabin property that opened last year. Everything at the bucolic, waterfront retreat runs on solar energy; they treat their wastewater on-site and provide organic, biodegradable products for their guests. The A-frame cabanas and floating platforms in the lagoon are all built with local materials — Ms. Calzada, who is from Tabasco and has a background in sustainable architecture, hopes that her property, and its success, will be an example to the local community and to future developers

“Why would you not want to take care of the lagoon and the land?” she said. “If you’re taking care of the lagoon, you’re taking care of your investment.”

It is, undeniably, Bacalar’s moment on the brink. An update to the region’s urban development plan, or Programma Municipal de Ordenamineto Territorial Ecologico (POEL), is in the works, the first since 2005. The POEL will determine where developers can build, how high buildings can be, how many rooms hotels are allowed to have. The plan will determine the direction of Bacalar and likely the fate of the lagoon, but it could be years before it is released.

Ms. Chomicz hopes that, in the meantime, tourists can help make a difference.

“Limit your waste by using refillable water bottles. Keep the party in the city, not on the lagoon. And ask questions!” she recommends. “Ask about the toilets: Are they connected to the system? How do they treat the wastewater? Ask your guides and boat operators. What do stromatolites look like? How can we take care of them?”

Aboard Yemaya, we cruised south to Bird Island, a nature sanctuary of dense mangroves that erupted in a cacophony of birdsong as the sun dipped behind a dark bank of clouds looming off the western shoreline. Jumping overboard into the bathtub-warm water, I watched their ominous approach, both as weather and metaphor.

The skies opened, and as the rain pelted down, the lavender-gray lagoon appeared to boil. The sun broke through the storm and a rainbow appeared, ending where I stood. And for a moment, it all seemed solvable — the plight of the stromatolites, the tenuous balance between loving a place and destroying it, my wry sense of too-obvious symbolism and my questionable role in it all. We were at the actual end of the rainbow!

There are no perfect choices, Ms. Chomicz had told me, but there were better ones. Trying to make them seemed well worth it.

Lauren Sloss is based in San Francisco and covers travel, food and music. Follow her on Instagram: @lsloss and Twitter: @laurensloss

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