For centuries now, the Río Camuy has been imposing its will on the soft karstic underground limestone to create this incredible system of caves, the world's third-largest. This park spreads over an area of about 10 miles long and has 17 entrances in the area between the towns of Hatillo, Camuy and Lares.

Over the years, the caves have been important shelters for indigenous people, home to millions of bats that help keep the island's insect population under control, and a source of fertilizer. But no modern explorers went to the trouble of making a thorough investigation of the caves until 1958. This was when Russell and Jeanne Gurnee and Bob and Dorothy Rebille accompanied José Limeres (a Puerto Rican doctor) into a Río Camuy sinkhole. The upshot of this trip was a suggestion that the government purchase 300 acres of land around the Río Camuy caves as a nature preserve. Later, the Speleological Society of Puerto Rico explored and mapped the caves, eventually proposing that the government develop the Cueva Clara de Empalme (at the site of the Cavernas del Río Camuy) for tourism, and in 1986 the attraction opened.

It is a good idea to call the park for local conditions (too much rain can cause closures or abbreviated tours), and arrive before 10:30 to avoid crowds or a long wait.

Your visit begins with a film about the caves at the visitors center. Then you take a trolleybus that follows a spiraling road down through the jungle into a 200ft-deep sinkhole to Cueva Clara de Empalme (Clear Cave Junction), where you take a 45-minute guided walk through the cave. Here you walk past enormous stalagmites and stalactites, and into rooms littered with boulders. At one point, the ceiling of the cavern reaches a height of 170ft; at another, you can see the Río Camuy rushing through a tunnel.

After leaving the cave from a side passage, you take another tram to the Tres Pueblos sinkhole, which measures 650ft across and drops 400ft. Forty-two petroglyphs that you can now inspect have been found in Cueva Catedral (Cathedral Cave).

The last tour leaves at 14:00 if you want to see all three areas, or at 15:45 if you want to see just one sinkhole.