It’s when a body of water has fresh AND salt water. The salt water is very dense and thus heavy, remaining on the bottom while the fresh water is on top. It results in a body of water (ocean, lake, etc) having water underwater.
The salt water is so dense that it looks like very foggy water when viewed at from the fresh water part.
Halocline is actually very dangerous, if divers go down into the salty part. Some forget (or are unaware, as in caves for examples) of the Halocline, emerge from the foggy salt water and believe they are OUT OF WATER, resulting in them taking off their helmets, only to drown.
But it is super interesting and really beautiful.
EXTRA: I don’t know for absolute certain, but I am quite sure that the “magic underwater water” in the videogame ABZÛ is also Halocline, since it looks very much like that
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