Gruta do Lago Azul(Blue Lake Cave) , Bonito, Brazil:

Gruta do Lago Azul (Blue Lake Cave)  The tour starts with a 10-minute walk to the entrance of the cave, which was discovered by a local Indian in 1924. In its interior, after descending 100 meters, it is possible to see the lake of crystal clear water and depth of approximately 90 meters, which makes it one of world’s biggest flooded caves.
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