Puerto Rico is home to one of the largest underground river-cave systems in the world, and the easiest way to explore the island’s subterranean world is at Las Cavernas del Río Camuy (Carr. 129, km 18.9, 787/898-3100 or 787/898-3136, Wed.–Sun. 8 a.m.–3:45 p.m., $10 adults, $7 children under 12, free for senior citizens over 75, parking $2). The park is a well-maintained, tightly run ship, and it’s a good thing. This place draws major crowds, including busloads of schoolchildren. Buy a ticket, browse the gift shop, and watch a 10-minute film (English and Spanish) while waiting for the trolley, which runs every 30 minutes.

Once aboard, you zip down, down, down toward the mouth of Cueva Clara. Along the way you pass a mind-boggling display of virgin tropical forest. African tulips, mamey apples, passion fruit, red ginger, bananas, begonias, ferns, and the tiniest, most delicate orchids you’ve ever seen are everywhere. Before you know it, you’re standing at the entrance to Clara.

The cavern’s natural opening has been preserved. Visitors enter to the right of it through a larger, artificially constructed opening. The path steeply descends, bottoming out just below the natural opening, through which the sun shines brilliantly, creating the sort of mystical scene that could inspire visions of hobbits and fairies. But farther down are even more magnificent sights as the cavern opens into a 170-foot room thick with stalactites and stalagmites, most notably the Giant Stalagmite, measuring 17 feet tall and 30 feet in diameter. There’s also a subterranean waterfall created by the Río Camuy, which runs through parts of the cave. In addition to bats, crickets, and spiders, the cave is home to a creature that is so rare that this is the only place it lives. It’s a microscopic crustacean called Alloweckelia gurneii, and it can’t be seen by the naked eye.

Other natural sights in the park include Tres Pueblos sinkhole, seen from a viewing platform; Cathedral Cave; and Spiral Cave, accessible by a 200-step staircase to its mouth. There are also an interactive miniature gold mine where kids can pan for “nuggets,” a snack bar, a gift shop, and trails. And although it’s not publicized, there are a limited number of wooded campsites.

Of course, the part of the caves the general public sees is a tiny fraction of the wonders to be found. Lucky for experienced spelunkers, they can arrange tours to explore more remote parts of the caves. Sturdy, nonslip shoes are required; the cave paths get very slippery.

Expediciones Palenque (787/823-4354 or 787/306-4382, info [at] expedicionespalenque [dot] com, www.expedicionespalenque.com, $90) offers a daylong adventure hiking, rappelling, caving, and body-rafting along the Río Camuy and into Resurgencia Cave.