As explained in the description of Cenote Kukulkan, cenote divers sometimes encounter a layer of saltwater that rests below the freshwater. Because they have different densities, the freshwater and saltwater layers stratify, much like olive oil and vinegar. The boundary between the freshwater and saltwater is called the halocline. “Halo” refers to salinity and “cline” signifies a gradient. The halocline leads to some interesting visual effects.
This photo of the halocline is in focus. Even though it appears blurry, it accurately represents what a diver swimming through a halocline might see. When salt and fresh water are disturbed by a diver's movements, they mix and create a blurry visual effect, analogous to the way that olive oil and vinegar look blurry when stirred together. Divers have compared the experience of swimming through a halocline to losing a contact, being very drunk, or swimming through Vaseline. Some divers enjoy the visual effects of the halocline, and some do not. Thankfully, a diver has only to ascend or descend out of the level of the interface to find perfect visibility once again.