PLAYA DEL CARMEN, Mexico — The Yucatan Peninsula in southern Mexico has attracted visitors since the Mayans had one of the Western Hemisphere’s most advanced civilizations in the third century. However, what’s under the Yucatan is what attracted me recently. For 2 million years, an underground river has flowed through a cave system that spider webs for 20 miles just south of this touristy beach town of Playa del Carmen.

The Rio Secreto Cave Tour

I did the Rio Secreto cave tour, one of the wildest river cruises in the world . The difference is you do it without a boat. It’s just you, a life vest, a wetsuit and a lighted helmet. That and a guide stood between seven of us being hopelessly lost forever 60 feet underground.

Group shot inside

A shuttle bus took us from Playa del Carmen to a big opening in the jungle where about 40 tourists were partnered up with Rio Secreto Cave tour guides sporting various styles of wetsuits. We got Alberto, a tall skinny Mexican with longish hair and a thin moustache. He looked like an extra in a movie about the Mexican Revolution.

The Secreto has been a major part of Yucatan for a millenium. The Mayans used this river for their water supply in the 1500s and it’s still drinkable today. It formed when rainwater carved out this maze of caves in the limestone rocks. More than 33,000 feet of caves have been discovered in Yucatan, most of them underwater and now the gravesite of many overly adventurous scuba divers. The Rio Secreto is only slightly submerged. It’s just enough to get chilled in the surprisingly cool 72-degree temperatures underground.

Inside the cave

When we reached the cave after a short bus ride, we walked down some wooden stairs to a black gaping maw in the rock. This was one of about 17 entrances to the caves. I peered inside and saw nothing but blackness. I asked Alberto if any of his tourists have had claustrophobia.

“Oh, yes. It’s a big problem,” he said. “They come here and take one look at the cave and turn around and walk back.”

Stalactites

Fortunately, I didn’t have a problem during our Rio Secreto Cave Tour, as we precariously crept our way down rocky stairs in river shoes with grated soles. The quarters aren’t real tight. However, the millions of stalactites make you feel like you’re walking under an armory of medieval swords. Stalactites are to caves what plants are to jungles. They’re formed when rain seeps through the limestone and the rock drips down to form sharp, long formations. When water comes up from underground, it forms stalagmites which are more round at the tip. When the stalagmites and stalactites meet, they grow outward. In Kentucky which has the largest cave system in the world with about 390 miles of caves, some of these columns are five stories high. Rio Secreto’s are more modest.

We waded through knee-deep water for a few hundred yards. Despite the slippery terrain we were able to negotiate the rocks on the riverbed as the crystal-clear water made them as visible as if they were above ground. This is arguably the cleanest river in the Western Hemisphere . Parts of it were so blue it looked like I was beachcombing in French Polynesia on a moonless night.

Sunbeam

Alberto was a wealth of information — and fear.

“Is anyone afraid of spiders?” he asked.

A tall, overweight American raised his hand.

“He’s terrified of spiders,” his wife said.

“But only if they have eight legs,” he said.

Alberto dipped his head to shine his helmet lamp on a rock. Dashing across was a spider with a long antenna waving in the air like a sea fan. He picked it up, and it dashed back under a rock.

“He’s hiding,” Alberto said.

“No, he’s not,” the American man said. “He’s plotting.”

Shadowy figures

We saw other spiders, some as big as Alberto’s fist. Fortunately, they only had six legs. Otherwise we would’ve had a very large, very panicky man on our hands trying to splash his way through a pitch-black cave half underwater. And it was pitch black. It was as black as the inside of a giant lump of coal. Alberto once had us turn off our helmet lamps and close our eyes. We counted to three and opened them. Have you ever been in a room so dark that you lose your sense of what’s up and what’s down? Imagine that waist deep in water. That’s how dark that cave was with no light. For a second I let my mind wander. What if …

… all our lights broke and we had to get back? How would we do it? There is a thin yellow rope that goes from the cave entrance all along the Rio Secreto cave tour route. They’re designed for divers to feel their way back without getting lost but they’re also for spelunking (That sounds oddly sexual but crawling underground in the water was the least sensual experience of my life).

The rock formations were really beautiful. The stalactites were a combination of white and gold and yellow and brown. Some of the rocks had a crystallized white coating on them like vanilla frosting. They call it moon milk. It’s when bacteria collects in the water and forms on the rocks it crystallizes.

These caves are also homes to cenotes, deep caverns in the rock that go hundreds of feet deep. This is where free divers have had competitions for a sport in which the world record is 706 feet. Some of them even make it up alive.

Our Rio Secreto Cave tour was completed in 90 minutes, so did we as a rain shower greeted us. Still, despite the rain, the air was curiously bright.