The ocean's high salinity comes from two main sources: runoff from the land and openings in the seafloor. As far removed as rocks on land may seem from the ocean, they influence the ocean's chemical composition, which includes its saltiness.

The high salinity of the water around the Florida Keys is driven by a combination of intense evaporation and unique geological features. Because the Keys are located in a subtropical climate with high temperatures and consistent sunlight, the rate of evaporation is very high; as water turns to vapor, it leaves the salt behind, concentrating the remaining liquid. Unlike areas near large river deltas, the Keys lack significant freshwater runoff that would typically dilute the seawater. Furthermore, the region is home to vast underground salt domes and thick layers of ancient evaporite minerals beneath the seabed. These geological deposits can slowly leach salt into the surrounding water. The shallow nature of the Florida Bay also plays a role; shallow water heats up faster, accelerating evaporation and preventing the salt from being quickly dispersed by deep-ocean currents. This high salinity creates a unique environment that supports specific marine life, like mangroves and seagrasses, which have adapted to thrive in such "briny" conditions.