I did some searching about sinkholes in Florida and found this one ...
it's 120' down and very very old:
http://www.floridastateparks.org/devilsmillhopper/
Quote:
Welcome to Devil's Millhopper Geological State Park
In the midst of north Florida's sandy terrain and pine forests, a bowl-shaped cavity 120 feet deep leads down to a miniature rain forest. Small streams trickle down the steep slopes of the limestone sinkhole, disappearing through crevices in the ground. Lush vegetation thrives in the shade of the walls even in dry summers. A significant geological formation, Devil's Millhopper is a National Natural Landmark that has been visited by the curious since the early 1880s.
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