The limestone labyrinth now known as Cueva Arenales formed over three million years as the Río Cibuco carved underground channels through the northern karst belt, leaving behind a network of galleries decorated with stalactites, fossilized coral beds, and pre-Columbian petroglyphs attributed to the Taíno people who inhabited the region before Spanish contact in 1493. The cave system's name derives from the adjacent Arenales ward, itself named for the sandy deposits left by seasonal river flooding that once made the area a critical agricultural zone during Puerto Rico's colonial period. Unlike the heavily regulated Río Camuy caves to the west, Cueva Arenales remained largely undeveloped until the 1990s, when local geologists documented its extent and ecological significance, noting colonies of Antillean fruit bats and rare ferns that thrive in the perpetual humidity. Today the site anchors a growing adventure tourism corridor that links the caves to Charco Azul, a spring-fed pool one kilometer downstream, and the Vega Baja Canyon, a vertical-walled gorge where the river narrows to less than three meters. The karst geology visible throughout the system offers a cross-section of the island's tectonic history: the limestone dates to the Oligocene epoch, when shallow seas covered much of what is now northern Puerto Rico, and the cave ceilings preserve fossil imprints of sea urchins and mollusks that settled on the ancient seabed. The petroglyphs etched into the cave walls near the main chamber depict anthropomorphic figures and concentric circles, motifs common to Taíno ceremonial art, and their placement suggests the caves served as a sacred site or refuge during the pre-colonial era. Guided expeditions through the caves gained traction in the early 2000s as outfitters based in Vega Baja and nearby Manatí began offering half-day packages that combined caving with rappelling, river trekking, and cliff jumping at adjacent natural formations. The tours traverse passages that range from cathedral-sized chambers to crawl spaces requiring helmet lights and knee pads, with water levels fluctuating seasonally. The northern karst belt's porous bedrock means that even light rainfall can raise the river by half a meter within hours, a dynamic that shapes both the ecosystem and the visitor experience. The caves remain free to enter without a ticket, though most visitors opt for supervised tours that provide safety equipment and context about the geology and cultural history embedded in the rock.