The underwater waterfall in Mauritius is a famous landmark that is an optical illusion. There is not really a waterfall.
Underwater waterfalls do exist in other places, but are not visible, because water is transparent and the darkness of the blue color is determined by its depth.
In Mauritius there isn’t any flow of water from higher depths to lower depths. There aren’t multiple sources of water that meet. However, there is a submarine plateau from which sand is sinking into deeper waters.
The illusion is simply your inferior temporal (IT) cortex not recognizing the image. You know what waterfalls look like and assume that this is an underwater waterfall.
