This is off the Island of Mauritius in the Indian Ocean! And actually it’s not really a waterfall, it’s the underwater sediment/minerals and sand that is being swept off the continental shelf into the epipelagic waters by some pretty strong currents, creating a sort of illusion that the water itself is rushing over into a waterfall. It’s a pretty cool illusion though!
There aren’t ever such things as underwater waterfalls or rivers that are just made out of water since water is technically all the same density (besides fresh and saltwater) and the only thing that separates it would be the salinity and/or the thermocline!
But when water is actually mixed with something denser, such as hydrogen sulfide or perhaps salt water crashing with fresh water, it can actually form underwater rivers such as the one in Mexico!
Edited by afrokock - Aug 08 2014 at 4:05pm