Gruta do Lago Azul (the Blue Lake Cave) is one of the numerous natural monuments of Brazil. Located in Bonito, Mato Grosso do Sul, it practically comprises of two caves the main feature of which are fragile calcareous formations. The cave has been included among the protected natural monuments of Brazil since 1978.
What makes Gruta do Lago Azul so special is its main hall with a floor, which slopes to a subterranean lake measuring 50 metres (160 feet) in length. The circular entrance is about 40 metres (130 feet) wide, allowing sunlight it to illuminate the lake giving interesting glitters to the water. Between September and February, the water takes on a mesmerizing intense blue shade, and hence its name – Lago Azul (Blue Lake). Fossil bones of large, prehistoric mammals that roamed the site over 12,000 years ago have been found here. Among these, giant sloths, armadillos and sabre-toothed tigers caught the eye of the French-Brazilian expedition who took a dive in the blue waters and came across the fossils in 1992.
To protect the fragile calcareous formations in the cave and the lake ecosystem, as well as the area around the cave, the National Institute of Historic and Artistic Heritage (IPHAN) declared the cave a natural monument and protected area. Visitors are allowed only accompanied by guides from the municipality of Bonito, and access to children between 0 and 5 years of age is forbidden, for safety reasons.
Gruta do Lago Azul is one of the largest inundated cavities in the world.
Gruta de Nossa Senhora Aparecida is the second cave of the group and unlike the Blue Lake Cave it does not contain water and it has very little natural light. It has a single, large hall with a sloping floor measuring 100 metres (330 feet) in its widest part. The hallmarks of this cave are the so-called speleothems, some of which have been called ‘angels’ because of their close resemblance to winged humans. For safety reasons, Gruta de Nossa Senhora Aparecida is closed to visitors. It has also been declared a protected natural monument and included in the protected natural patrimony of Brazil in 1978.