The tourist tram threw me, I have to confess. I had images of donning a grubby coverall, hard hat, and headlamp, and then wriggling through a pile of green guano to enter the cave. But entering the Cueva Clara de Empalme, which is the largest cavern in the Rio Camuy network, is as simple as stepping off the trolley and walking down the cement path. At first I was a little put off, thinking that following a set of handrails on a guided tour of a cave would be unadventurous. But it wasn't. The instant we set foot inside, I was glad for the walkway, for the cave was a vast assemblage of stalactites, stalagmites, and sudden descents farther and farther into the earth. The air no longer bore the pungent whiff of jungle but was still and dusty. Sunlight filtered in through the cave's entryway, giving the inside a ghostly feel. It was as if we had not descended into the earth but somehow had stepped onto another planet. The chatter among the tour group stopped as we gaped upward at the ceiling, hundreds of feet high. The only sound was that of rushing water, unseen but very close, beneath our feet.

What must it have been like, I wondered, for Russell Gurnee to explore this cave back in the 1960s? It was Gurnee who undertook the systematic mapping of the cave system and the thundering Rio Camuy. The entire cave network stretches for miles, formed by an endless continuum of galleries and passages. Most of it is off-limits to the public, and much is still uncharted, but the Cueva Clara de Empalme and the nearby Espiral sinkhole have ample magnificence. It boggled my mind to hear the tour guide talk about the composition of stalactites and stalagmites, none of which we were allowed to touch. Each foot of growth on those rocky spikes, some of which were dozens of feet tall, represented 1,000 years of water droplets slowly calcifying until they hardened into rock. Even more mind-boggling was the notion that the caves are 45 million years old.