|karst cave Doline blue hole
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The Ocean Hole is a strange geologic sight, a saltwater lake located inland, about 300 m from the shore. The explanation is simple: an underground cave system is filled with sea water. Wherever it collapses the sea water becomes acessible from the surface.
The hole is a circular shaft with steep limestone walls, a collapsed cave. There is an overlook at the northern side of the hole, where the road ends with a parking lot. The place also has a plate which explains the specialties of this place. A trail is leading down some steps to the shore of the sea water. This is really interesting, as the water is full of fish. The hole is very popular among divers, which descend into the 30 m deep hole.
The cave systems in the Bahamas islands are often waterfilled, which is a result of the last Ice Age. The limestones were karstified during the last cold period, when the sea water level was 100 m lower. Draining to this level, the caves formed ending at around this ancient sea level. When the ice caps melted, the sea level raised and the caves were flooded. Many of the blue holes are caves in limestone areas which are now completely underwater. The inland dolines are similar to blue holes from the geologic term, the only difference is that the surface is not covered by sea water.
Underwater cave systems on such small islands can contain sweet water, salt water, or brackish water. This depends on the size of the caves and the amount of rain on the island. If the amount of rain is low, the sweet water will form a water body underground, which is floating on top of the salt water because of its lower specific weight. When the amount of rainfall is high, the sweet water will use the caves to flow into the sea, an underground drainage system as known from classical karst. And if its inbetween, and the caves are rather big, it may even mix and form brakish areas underground. In this case it seems Eleuthera is too small and has too little rain, so the underground caves are completely filled wit salt water.