Guillermo D. Christy is standing at the edge of a cenote when we speak. He has just returned from France, where he collected his daughter from her university studies, and he sounds like a man briefly at peace with the world. But peace, in the Yucatán Peninsula, is hard to hold onto for long. He looks out over the water – still, clear, and ancient – and starts to talk about what is happening beneath it.

“We’re in real trouble,” he says. “Twenty-nine per cent of the cenotes near Playa del Carmen are contaminated. A lot of E. coli, a lot of coliforms. The government’s own data tells us that.”

Christy is an engineer – a water treatment specialist who has spent 25 years working with hotels along the Riviera Maya, analysing what went in and trying to understand what came out the other side. Over those decades, the question that consumed him was always the same. Where does the water go? What happens to it underground? The answers he found brought him to where he stands today. Because Christy is at the centre of one of the most urgent environmental campaigns in Mexico, running on no salary, sustained by a community of volunteers and fighting a battle that the peninsula’s booming tourism economy would rather turn a blind eye to.

Beneath the resorts and the highways of the Yucatán Peninsula, beneath its ancient cities and its jungle floor, lies one of the largest underwater cave systems on Earth. Rainwater, slightly acidic, has spent millions of years filtering down through limestone bedrock, dissolving it layer by layer, carving tunnels and chambers and cathedral-like caverns across the entire peninsula. The result is a freshwater aquifer fed by thousands of openings in the surface.

These are the cenotes. Natural collapses in the rock. For the Maya, cenotes were sacred. They were, the ancient Maya believed, portals into the underworld “Xibalba” itself, where offerings of jade, gold, pottery, copal incense and human life were made to the gods who controlled rain and fertility. In a landscape with no surface rivers, no lakes and no visible source of water, the cenotes must have appeared miraculous – openings in the earth through which the world below could communicate with those above. The Maya did not simply drink from them. They listened to them, enjoying a spiritual relationship that endured for centuries.

The more Christy talks about modern sensibilities, however, the more apparent it becomes that what has replaced it – across a peninsula where the majority of residents were born elsewhere, drawn by the promise of tourism work – is something closer to indifference.