This sinkhole was formed naturally when the ground over unstable layers collapsed on itself. There is an underwater tunnel leading to the sea 500 metres away, leading to a mixing of fresh and sea water in the hole. It is possible to dive this, however due to possible extreme tidal conditions, caution is advised.
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Jacobs Well in Wimberly Texas where my family is from. One of the longest underwater spring cavern in the US
Jason de Caires Taylor’s underwater sculpture garden in Cancun. "Constructed out of concrete and steel, and bolted to the ocean substrate, the works here act as artificial reefs that provide “an ideal habitat for filter feeding organisms.”" www.dedeceblog.co...
Unbelievable....Austria's Green Lake in the Hochschwab Mountains is a hiking trail in the winter. The snow melts in early summer and creates a completely clear lake. The lake has a grassy bottom, complete with underwater trails, park benches, and bridges. So cool!