This phenomenon is called a halocline, in which fresh and salt water meet causing a sharp change in the salinity gradient of water. Haloclines are commonly observed in underwater caves located near the ocean. Fresh water originates from land creating a layer overlying the dense salt water accumulated from the sea. This causes an optical illusion in which the underwater cave appears to contain an unexplainable air cavity.
[deleted] on June 9th, 2017 at 13:33 UTC »
Also : https://en.wikipedia.org/wiki/Halocline
C4RB0NUN1T on June 9th, 2017 at 14:12 UTC »
They have these in Bikini Bottom as well.
StuffyUnicorn on June 9th, 2017 at 14:28 UTC »
This relevant scene from Planet Earth Caves is still one of my favorite "woooooahhhhh" moments in the series