The first thing you notice as you descend beneath the jungle floor is the clarity of the water. Our headlights pierce straight through the underground river to the calcite bottom, at the same time that they illuminate a million stalactites and stalagmites (I finally learned how to distinguish these words - stalagmites has a "g" for ground, so they are the ones that grow upwards). The Rio Secreto Nature Reserve  is only a few minutes drive south of Playa del Carmen and is a popular half-day tour.
When we arrived, we were taken in a van deeper into the forest, then asked to strip down, shower off all body oils, sunscreen, and bug dope, and don wetsuits, booties, and helmets to hike to the cave entrance. There an older gentleman, Don Manuel, lit a bowl of copal incense and bathed us in its sweet smoke as he made an invocation in the Yucatec Mayan tongue, asking that we be allowed to enter xibalba, the underworld and, more importantly, that we be allowed to exit, which we did, ninety jaw-dropping, clear-water-dripping minutes later.
Photo by Gerardo Chaves