If you're a diver, you should visit Cenote Angelita, Mexico. There's a cave there. If you dive to depths of 30 meters, the water is a fresh water, but if you dive to depths of more than 60 meters, the water turns salty, there you can see a 'river bank', complete with trees and foliage leaves. But of course, it's not a common river, it's a layer of hydrogen sulfide, but it looks like a river ... amazing isn't it?