this is the famous blue undergound lake in the chapada. the lake sits inside an underground cave and is lit by a single shaft of light which illuminates it creating the most amazing blue shimer. you can dive in this lake. i did and it was spectacular, if a bit scary. its quite an amazing scene to witness in person. there are many lakes in this complex of caves, this one being the most famous because it receives the most amount of light, and thus creates the most amount of colour.