About an hour and half from the city of San Juan, far from the sandy, Caribbean beaches that lure most visitors to Puerto Rico, a network of more than 220 subterranean caves stretches 286 acres below the bustle of the island’s heavily trafficked thoroughfares.

The Cavernas del Río Camuy is the third largest cave network on the planet, and the largest in the Western Hemisphere. The Río Camuy (Camuy River) shaped the stalactites, stalagmites and sinkholes when coursing through the area more than a million years ago.

Researchers unearthed the Río Camuy cave system in the late 1950s, though it was likely a re-discovery. Archaeologists believe Puerto Rico’s native population explored the area centuries before. Today, more than 10 miles of mapped trails run through the caves, with close to 20 entrances and additional, smaller cave networks that researchers have yet to probe.

In contrast to the lush, surrounding forest of the Parque de las Cavernas del Río Camuy, the main entrance to the caves is cool, dark and gaping. The noise from above disappears with each step, and is replaced by the echoes of dripping water. Portions of the guided trail are darker than others, but there are soft lights and railings along the relatively flat path.

In the Cavernas del Río Camuy, limestone and subterranean waterways ensconce an underground world that is home to multiple bat species and the Alloweckelia Gurnee, a type of blind fish that exists only in the caves’ waters. The most massive visible chamber is the Cueva Clara, which at 180ft could fit a 20-storey building inside. Though knowledgeable guides lead multiple tours through the caves during visitor hours, the dense walls maintain a peaceful quiet.

The tour through the caves takes about an hour and is appropriate for children and adults. Booking ahead of time is advisable — tours fill up quickly and the park caps daily attendance at around 1,500 visitors — as is wearing shoes with traction for walking in the damp caves. Picnic areas and walking trails are located throughout the park – a good way to adjust to the sunlight again after spending the day submerged in Puerto Rico’s most hidden natural landscape.