One of Bonito’s showcase attractions, the Gruta do Lago Azul Natural Monument does ample justice to its name (Blue Lake Grotto). This vast cavern contains an underground lake that, when illuminated by the sun’s rays, takes on an otherworldly hue of blue. This phenomenon is the most intense in the mornings in December and early January.
While nobody knows the depth of the lake (285 feet, or 87 meters, is the maximum depth reached by divers), the cavern is believed to date back 10 million years based on prehistoric animal fossils found on site. The mildly alarming descent, a staircase of 294 irregular steps lacking railings, allows for the viewing of some sculptural stalactites and stalagmites.
The Gruta do Lago Azul can be reached by following the Rodovia Três Morros from Bonito for 14 miles (22 km), 11 (18) of which are along a dirt road. The site is open daily from 6am to 2pm, and entry costs R$36.