Blue Lake Cave, Natural cave system with underground lake in Bonito, Brazil

Blue Lake Cave is the main cave of the Gruta do Lago Azul Natural Monument in Bonito, Mato Grosso do Sul. Its circular entrance, about 40 metres across, lets sunlight fall on a sloping floor that descends to a subterranean lake more than 50 metres long. The monument also holds the dry Gruta de Nossa Senhora Aparecida, whose single hall reaches 100 metres at its widest and contains speleothems called "angels" for their winged, human-like shapes.

The cave system formed over millions of years as water carved through limestone, creating the underground passages and lake. Archaeological findings near the cave reveal that ancient animals once inhabited the region during different climate periods.

The name comes from the intense blue the water turns between September and February, when sunlight entering the 40-metre circular opening reaches the lake. Visits take place only with guides from the municipality of Bonito, under rules drawn up to keep the fragile limestone and the aquatic ecosystem untouched. The neighbouring Gruta de Nossa Senhora Aparecida is closed to visitors for safety reasons.

Entry to the cave is only possible with certified guides who maintain conservation standards during the visit. Tours are available year-round, though certain seasons offer different viewing conditions and experiences.

The lake's blue colour appears between September and February, when sunlight entering through the circular opening, about 40 metres across, reaches the water. The same cave floor has yielded fossil bones of giant sloths, armadillos and sabre-toothed tigers from the Pleistocene.