The Amazon is one of the most impressive feats of natural beauty that this world has to offer, so it should come as no surprise to learn that people dream of traveling to this rainforest and seeing the river for themselves. However, you wouldn't want to do this without a guide - and you certainly wouldn't want to go swimming at Rio Negro Beach. Not unless you want to be eaten by a piranha, anyway.
Not only is the water around this beach black in color, but the red-bellied piranhas have no problem attacking humans with little provocation. So, it might just be a good idea to avoid the water altogether.