In many coastal caves there is a fresh layer on top of a dense layer of salt water. One could expect mixing of salt and fresh water in a cave and the formation of a layer of water of medium salinity. Instead, imagine the relationship between salt and fresh water as an oil-water relationship – they never mix because of different densities.

In most cases, the so-called halocline is formed at the junction. The glycolin usually has clear boundaries (a few centimeters in width), but it can be more blurred and spread over several meters. Cave divers can see the boundary of the halocline because of the refraction of light due to the different density of water. It can look like a mirror surface, a false bottom or a surface of water, and when you touch the halocline begins to blur. The unique chemical composition of this layer of water is discussed in more detail in subsequent publications.

With the help of reconnaissance, accurate geodesy and cartography, cave divers have brought many new knowledge to the scientific community. Without this information, it would be much more difficult to model the flow regimes for better understanding them. Cave divers can help hydrological research by measuring the speed of water flow, and help to investigate the quality of groundwater, noting the color of water, temperature, depth of the halocline, and making water sampling.