An Underwater River

 
If you are a professional diver you should visit Cenote Angelita, Mexico. There's some kind of cave. When you go deeper than 30 meters the water is fresh, deeper than 60 - it's salty. And then you see a river underneath, with trees and leaves. But of course, it's not a real river. It's just a layer of hydrogen sulphide. But it looks amazing. (Source)

(Click map to enlarge - Map Source)

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