Río Camuy Cave Park is one of the most impressive natural attractions in Puerto Rico and one of the larger cave systems in the Western Hemisphere. Located about 25 kilometres southwest of Arecibo, the park is built around a section of the underground Río Camuy river system, which has been carving through the limestone karst for millions of years.
Tours begin with a trolley ride down into a sinkhole, then continue on foot through lit chambers decorated with stalactites, stalagmites, and cave formations. The main cave — Cueva Clara — is enormous, its ceiling lost in darkness above. The park is well managed and the guided tours are informative, though it can get crowded on weekends. Go on a weekday morning if you can, and check that the park is open before you make the trip — it closes periodically after heavy rain.