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.