A trick of light and geology
Inside Chapada Diamantina National Park, a geological window opens for a few hours a day, allowing a single beam of sunlight to enter the Poço Encantado cave. From April to September, this sunbeam penetrates the darkness and strikes the surface of a submerged pool, creating an intense, blue glow. The effect happens because the light refracts off a high concentration of minerals, particularly magnesium, dissolved in the water. The pool is an immense sunken chamber, 110 meters long and 70 meters wide, filled with groundwater.
The water's extreme transparency is a product of its chemistry. Rainwater seeps through the region's Proterozoic-era carbonate rocks, a process which filters out sediments and algae. This natural filtration makes the water so clear that ancient tree trunks and rocks are visible on the bottom, approximately 60 meters below the surface. This unique environment also contains a species of endemic troglobitic (cave-dwelling) catfish, Rhamdiopsis krugi, which has evolved without eyes or skin pigmentation.
Geological history
The Poço Encantado is a karst feature, a landscape formed by the dissolution of soluble rocks like limestone and dolomite. The cave systems in the Chapada Diamantina region began forming over 66 million years ago as water eroded the carbonate rock, creating vast underground networks. The main chamber of Poço Encantado is a large breakdown room, formed by the collapse of the ceiling over geological time.
The cave shows the local water table of the local water table. Its water level fluctuates by about 1.5 meters throughout the year. Because of the delicate nature of the pool's ecosystem and the need to preserve its clarity, swimming has been prohibited since the 1990s. The nearby Poço Azul, which exists in a similar geological formation, allows swimming as it is fed by a flowing underground river that prevents stagnation.