Between two liquids of different densities, always an interface forms in between. Just imagine an oil and vinegar dressing. In the Yucatan aquifer, a similar interface permenantly exists between the fresh and saline waters and is called the halocline (from the APSA Cavern guide manual, section hydrogeology) as this is where there is a strong gradient of change in salt concentration. It is a wonderful experience being able to see and play with the halocline in some cenotes and caves! You can see it with your eyes because some of the beams from your light are refracted, or bounced off the halocline surface. This is also why halocline may look different from above and below as the angle of reflection is different depending whether you are looking from the fresh or saline water into the other.
Cenote Eden for example has a very striking halocline where the thickness is less than 1m, however, much thicker haloclines spanning many metres are common, for example in Mayan Blue’s B tunnel.
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