"If the Yucatan Peninsula doesn't have any rivers, then where does its freshwater come from?"
It's a good question and one that provoked thought amongst my travelling companions when posed by Manily, our Mayan tour guide, as we bounced along a dirt road in a minivan, penetrating deeper and deeper into dense verdant jungle 30 minutes inland of Playa del Carmen's sun-soaked beaches.
The answer, continued Manily — who, to my bemusement, spoke fluent English with a unique Cockney-Mexican accent he'd acquired living in Britain for nine years (in addition to impressive French and German) — is found in one word: cenotes.
A cenote is a deep, natural sinkhole formed as rainwater infiltrates slowly through porous ground, eventually creating a sub-surface space in the soft limestone bedrock that is characteristic of Mexico. Some cenotes are connected to expansive cave systems that may stretch for dozens of kilometres. The Yucatan's native Mayan people considered these sinkholes as sacred gifts from the gods. The Spanish who colonized the region derived the word cenote from the Mayan dzonot, which refers to any location with accessible groundwater.
Disembarking at the Rio Secreto entrance, we were greeted by our cenote guide, Fabio. This particular cenote, he explained, was discovered a few years ago by a Mayan chasing an iguana on his land. Adding to the region's growing adventure activities, the landowner decided to lease out his land to the Rio Secreto tour operator.
Formed over centuries by an underground river, the extensive maze of passages, tunnels, channels and dead-ends constitute the longest known partially-flooded cave in the Yucatan Peninsula. Unlike fully submerged caves, which demand technical cave-diving expertise, semi-sunken caves can be explored by the average person. More than 12 kilometres of Rio Secreto's semi-sunken passages have so far been mapped out, including 15 natural outlets located in different areas.
After squeezing into "shortie" wetsuits and floatation jackets, and capped with headlamp-equipped helmets, we followed Fabio along a dirt path winding through a tangle of tropical trees and bushes. Reaching a staircase made of giant stones where a mini-shrine offered thanks to the gods, we stepped down into the cenote entrance, which opened like a gaping mouth framed by a ruff of ferns.
The Mayans, Fabio explained, believe in three levels of the world: the sky, the earth and the underground.
"So, when we go underground in the cenote, we die," he said. "If the gods are happy with us, we are reborn."
Stepping onto a rubber mat that protected the dirt floor from our feet, we were soon floating in water so impossibly clear I could count the wrinkles on my toes. In order to protect the natural environment, we'd been required to shower before entering. We were also instructed not to touch anything.