
The underwater river - chanux
http://seawayblog.blogspot.com/2010/02/cenote-angelita-underwater-river.html
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cskau
Sort of same phenomenon actually happens in the ocean if I remember correctly.

When I was taking my diver's license some years back we were told that
temperature differences in different water flows might also cause a visible
layer between the two such flows.

The underwater world is amazing.

