Threads from the Web of Life: Stories in Natural History

The Secret of the Cenot

4 The Secret of the Cenotés DESPITE the rain forest setting, the days can be hot and dry here in the Yucatan. The tropical rains do not linger in ponds and streams but percolate straight down through fractures in the limestone basement rock and disappear. No rivers cut misty canyons in the forested plain—the rivers are hidden, flowing underground beyond the reach of roots descending from above. The soil will not support trees that soar through layered canopies—as the less porous soils do farther south. Here, unshaded patches of ground bake in the noonday sun, adding a thirsty heat to the understory when the trade winds are oppressed by summer’s doldrums. But the denizens of this forest know of a respite from the midday tropical tedium. Every trail through this jungle, if followed far enough, eventually leads to a hidden oasis. From a distance these appear as nothing more than sun gaps in the jungle. However , closer approach reveals that the clearings are in fact great depressions, with the sheer faces of their opposite walls coming into view as the near edge approaches. They are great circular wells, some more than one hundred feet across—and deeper than they are wide. These are cenotés. Calm pools fill the bottoms of the columnar depressions, as far as two hundred feet down. The pool may be luminescent turquoise blue or algae green, depending on the particular subterranean river it reveals. The temperature in the jungle climbs as the day lengthens, but the moonpool at the bottom of the cenoté retains the crispness of the night. The sun shines directly into the bottom of the chamber only from the zenith; most of the time the pool remains concealed in the shade of its own cavernous cliffs, lit only by cool, blue skylight. Springs emerge halfway down the vertical The Secret of the Cenotés 51 walls and spill chill waterfalls through the air to patter onto the surface. Iridescent Motmots, blue-green tail feathers streaming behind them, chase each other across the water, their calls echoing through the chamber. Ferns, vines, and orchids growing out from ledges are reflected in silhouette on the still surface. Their flowers reaching from the walls glow backlit in sunlight filtered through treetops hundreds of feet above Beneath their tranquil beauty the cenotés mask a most violent past. The stillness that imbues these sunken oases belies a fiery event long ago during the age of the dinosaurs that was preamble to their creation. The mammals who lived in hiding then are now the dominant animals in the surrounding jungle. But they remain as quiet as they were when they lived in fear of the domineering reptiles. The calls that ring through the trees here are still those of the dinosaurs, in the feathered form in which they have descended to the present. When the dry season compels the largest of the mammals— the tapir, the jaguar, the peccaries—to search for water, they return to the cenotés. They step over the rim to find a narrow trail that begins in roots protruding through the thin layer of topsoil, then spirals downward, pressed into the rocky limestone walls. At the bottom the trail flattens out onto strata that were once the bed of a tropical sea. In between, for most of its height, the descent is a slippery traverse over compacted rock and sand. Vertical walls generally read long geological times in short distances. A foot of descent often represents thousands of years of sedimentary accumulation. But one day here long ago saw an exception to that rule. Hundreds of vertical feet of rock were deposited in only a matter of minutes, put in place by a flying avalanche sweeping across what would one day be the Yucatan. THE dinosaurs witnessed the cataclysm that led to the creation of these cenotés many millions of years ago, though none of those present would survive the day to remember it. On the evening of the momentous event the coastal hadrosaurs had gathered on the bluff overlooking the sea. Their dark forms were cast in silhouette by sunset colors reflected from the water. As they found their places, each straightened and grew still. They held their stance erect, birdlike, calm but alert. Though they stood 52 Threads from the Web of Life twenty feet tall, they nonetheless fit in easily among the...