Puerto Rico's fringe areas are its least-known attributes. Tourists, in particular, have long viewed bustling San Juan as symbolic of all things Puerto Rican. Travelers throng to its colonial section, with its shops and restaurants and world-famous El Castillo de San Felipe del Morro, the fort that kept enemies at bay for almost four centuries. It is a city famous for its rum and nightlife, making it a perfect stopover for the cruise-ship legions meandering through the Caribbean.

But there is far more to Puerto Rico, a quiet and adventurous side hidden deep within its interior. There are canyons, mountains, white-water rivers, and places like the Caribbean National Forest. Also known as El Yunque, this dense jungle rain forest is riven by waterfalls tumbling off black-granite cliff faces. It is a place where parrots flit in the canopy and where the air is thick with the oxygenated scent of rampant vegetation.

If the island's charms could be peeled away by layers - first raucous San Juan and the hundreds of miles of pristine beaches and then the rain forest of the rugged interior - you would find a third layer that comprises a pair of secret gems that the island reveals to very few. Ironically, those jewels transcend the island itself, one reaching out into deep space and the other down into the bowels of the earth.

It was the pair of treasures - the Arecibo Observatory, so vast that space-shuttle astronauts can see it from outer space, and the Rio Camuy Caverns, the third-largest caving network in the world - that lured me to Puerto Rico. Not that I am a caver or an astronomist, but something about these places forced me into a paradigm shift. When I think of Puerto Rico, I think of rum and the fort. And maybe of West Side Story. The cave and the observatory had a certain virtue, for lack of a better word. Their timelessness and what they had to say about the earth and mankind challenged my impressions of Puerto Rico. For this adventure, I flew to San Juan, took an ocean-view room at the Embassy Suites Dorado del Mar Beach and Golf Resort, and then drove west at the crack of dawn, bound first for the Rio Camuy Caverns.