Your eyes usually adjust when you're plunged into darkness, and after a few moments you're able to distinguish familiar shapes - there's a lamp; there's a chair.
Not so at Rio Secreto, an underground system of rivers, pools and caves in the heart of Mexico's Riviera Maya, near bustling Playa del Carmen. When I switch off my headlamp after nearly two hours of spelunking 60 feet underground, I am quite literally in the dark, enveloped in an inky blackness that feels positively primeval.
The day begins as the tour group is met in a parking area only five minutes by car from Playa and loaded onto a blue monster truck. We're given the lowdown on Rio Secreto by our driver, Horacio, as we trundle 20 minutes through the thick jungle to the cave's entrance. First and foremost, he says, our group has to shower before entering the system - no sunscreen, no bug spray, no perfume allowed in the water - I guess that means all the product in my curly hair too. Then he gives a brief history of the cave's discovery, which I suspect is to be taken with a very large grain of salt. One day a few years ago, the property's owner, a native Maya, went hunting for an iguana to cook for lunch. His lizard du jour, however, had other plans and scurried down a hole, into which the owner promptly stuck his arm, and voilà, a portal to the underworld was revealed. Realizing he'd found something special, the forward-thinking farmer called Alltournative, an eco-tour company that has since mapped seven miles of underground caves, rivers and fresh-water sinkholes, called cenotes.
"This is tarantula season," says our guide, Kris, as he displays a hairy specimen the size of a saucer during our pre-tour briefing. I wonder - can they swim? We suit up in shorty wetsuits, life jackets, water shoes and pith helmets and march single file down a leafy-green path to the entrance of the cave as "Heigh-Ho," from Snow White and the Seven Dwarfs, begins a continuous loop in my head. We're taking a half-mile route that begins in a dry cave, where the air feels easily 10 degrees cooler than at the surface. As we begin our descent into the cave's belly, Kris, trained in both freshwater ecology and speleology (the study of caves), explains what we're seeing.
Bats make their homes in caves, and with them come ubiquitous mountains of bat guano. Kris pauses before one of these mounds to show us something remarkable. Since tours started last year, guides have stopped at this particular bat bathroom many times (why escapes me), each time pausing to shine a flashlight on it. Over time, 
the artificial light has triggered photosynthesis in a seed buried deep within the guano, and now there's a tiny green shoot growing out of the mound's peak. I'm struck by the tenacity of life, even in an environment like this, 60 feet underground and with no sunlight at all.
After about 40 minutes of leisurely hiking, we encounter the water, first lapping at our ankles, then deepening to our shoulders as we wade into the river. The expedition photographer scurries ahead and turns on the few stationary lights installed in the caves so that the group can pause for photo opps. We carry on, bobbing like corks along the river and through low-ceilinged passages, checking out the "shore," where huge stalagmites and stalactites meet in midair, creating columns of limestone thousands of years old.
When we reach a cavern the size of a high-school basketball gym, Kris turns off his powerful flashlight and asks us to do the same with our headlamps. Instantly we're plunged into total darkness and silence, save for the echoing sound of dripping water, as we float for a few minutes in a pool whose bottom we cannot touch. I can't even see my own hand as I wave it just inches in front of my face. Far from being scared, I feel peaceful and calm; the chatter in my head quiets as I listen to my own shallow breath. When Kris flicks the flashlight back on, it's a shock to see how close other people were the whole time - the quiet darkness was so deep that I felt I could've been there alone. Finally, we swim through the cave's narrow throat and emerge into the sunlight, blinding now after the pitch-blackness from which we've come.
Above ground, we're treated to an alfresco lunch of tamales and tamarind juice prepared by local women and narrated by Kris, who wants to make sure we understand the importance of Rio Secreto, both as an eco-tourism destination and as a touchstone of Maya culture. The day ends as we climb back on board the blue truck and lumber down the gravel road, away from the secret river and back to the topside world, all the richer for knowing now what lies beneath.
Rio Secreto Tours' three-hour excursions include lunch and transportation to and from the cave's entrance, for $89. If you have your own ride, it's $49 per person.
800-985-2664; riosecretotours.com
Related Content
It doesn’t get more relaxed than the boho boondocks of Tulum (a 90-minute drive south of Cancun). Check out these tips to enjoy the ultimate beach-bum hangout.
According to the Maya Calendar, the world will end this year. An unlikely pilgrim resolves to experience Tulum before the it does—or more specifically, before the spell of German pop-up the Papaya Playa Project does. DJs, hipsters, designers, and shamans all share the Mayan shade until May 5th, 2012.
Few places in the world boast as many idylic settings for golf as the Caribbean. With seaside holes replete with alluring vistas, you're going to want to schedule a tee time in paradise.
