Marathon, Florida, United States

Lost Blue Hole

Lost Blue Hole is a circular karst sinkhole on the outer shelf of the Middle Florida Keys south of Marathon, formed during Pleistocene low-sea-level periods when the limestone shelf was exposed and groundwater carved it out from below. Sea-level rise then drowned the feature. The rim sits at about 22 metres below the surface and the floor is at 40 metres, with vertical walls of bare limestone descending into the dark. Currents above the hole are typically moderate, but inside the pit the water is still and almost always crystal clear. The site attracts large pelagic predators: Caribbean reef sharks patrol the rim, occasional silky and bull sharks pass through in summer, and large barracuda and goliath grouper hold inside. The walls hold deep-water sponges, gorgonians and small caves where Caribbean spiny lobster gather. Advanced certification, deep training and good gas planning required; the open-water dive is reasonable but going to the floor demands deep certification.