"Parque Nacional de las Cavernas del Río Camuy, a natural limestone cave system, is a family-friendly natural wonder. And the Río Camuy that runs through it is the third-largest underground river in the world. It's a grand experience that provides a lot of photo ops, particularly Clara Cave, which you need to visit via a ticketed trolley tour. If you're visiting Puerto Rico for the island's outdoor sights and adventures, don't skip this." - Alicia Kennedy