THE LINE OF BEAUTY

A controversial new railway is making it easier to explore Mexico's Yucatán Peninsula-and with it, change is coming. Simon Willis follows its route to explore the region's perfectly preserved colonial towns, lush nature preserves, and archaeological wonders.

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DAWN was breaking over the lagoon when I looked down and saw one of the oldest life forms on earth.

It wasn't much to look at. Lumpy, white, and sitting just below the surface of the water, it resembled an overgrown cauliflower. But its appearance notwithstanding, this was one of the most remarkable objects I had ever encountered.

“It’s called a stromatolite,” my burly and ebullient guide, Edwin Ruiz, explained as he nursed his flask of morning coffee. This one, he continued, was probably around 12,000 years old. The first stromatolites lived 3.5 billion years ago: the organism is a living fossil from the planet's earliest days.

Ruiz was leading me on a sunrise kayaking tour of Lake Bacalar, a long, finger-like lagoon in southern Mexico near the border with Belize. Nicknamed “the lagoon of seven colors,” in the sunlight it occupies that blue-green part of the spectrum in which turquoise shades into teal, cerulean into cyan. The stromatolites, which are formed by colonies of microbes that metabolize carbon and nitrogen in the water, help keep the lagoon pristine. That morning, it was so clear that paddling through it felt like floating across a sheet of Saran wrap.

Bacalar was the southernmost stop on a 10-day trip around the Yucatán Peninsula organized by Journey Mexico. At that early hour, my wife, Charlie, and our 18-month-old son, Leo, were both still sleeping in our lakeside hotel, Boca de Agua.

In recent years, Bacalar has emerged as a serene alternative to busier and better known places like Tulum, farther north along the Caribbean coast. But now the lake, and the nearby town of the same name, are on the cusp of great change.