The Tanama River disappears beneath the karst plateau of central Puerto Rico at a geological threshold scoured by thirty million years of dissolution. The entrance, a vertical swallow known locally as Sumidero Tres Pueblos, drops twelve meters into a lightless chamber where the river resumes its course through a honeycomb of limestone galleries extending more than four kilometers underground. The water temperature holds steady at twenty-one degrees Celsius year-round, fed by aquifer springs insulated from surface heat. No raft, no boat—body rafting through the Tanama cave system means committing your weight to a buoyant vest and allowing the current to carry you through passages so narrow your shoulders graze the walls. The Tanama is part of the Río Abajo karst belt, a band of soluble rock that runs east to west across the island's interior. Spanish colonial surveyors documented the river's vanishing act in the seventeenth century, but the first recorded traverse of the subterranean section did not occur until 1973, led by a team from the Puerto Rico Speleological Society. The cave ecosystem supports populations of the Greater Antillean long-tongued bat and blind cave shrimp, species adapted to total darkness and nutrient-scarce conditions. Above ground, the forested buffer zone is managed as the Bosque Estatal de Río Abajo, a reserve protecting one of the last contiguous tracts of subtropical wet limestone forest in the Caribbean. Modern tanama river body rafting tours depart from Utuado, a municipality fifteen kilometers south of Arecibo whose economy pivoted from tobacco cultivation to adventure tourism in the late 1990s. Licensed operators provide wetsuit, helmet, and headlamp; the journey through the cave typically spans two to three hours, depending on water level and group pace. Participants descend fixed rope into the entrance chamber, then float downstream through a sequence of rooms separated by low-ceiling crawls and brief swimming sections. The rock overhead is pockmarked with scalloped erosion patterns and flowstone deposits, evidence of the river's patient work. Natural light penetrates only the first thirty meters; beyond that threshold, the world reduces to the beam of your headlamp and the sound of water moving over stone. The river emerges on the north slope of the plateau, rejoining surface flow near the community of Angeles. Puerto Rico's strict caving regulations, enacted in 2004, mandate guide certification and group size limits to protect both visitor safety and the cave's fragile biota.