The beautiful Cherokee sinkhole has been a silent witness of neglect by people, who have used it for swimming and... littered it with garbage. Happily the attitude towards the sinkhole is improving lately.
Cherokee Sink is a rounded jade green pool, 90 by 50 m large, surrounded by 0.5 – 9 m high cliffs. It has steep sides under the water as well and is up to 24.4 m deep. Sinkhole has formed by a collapse of cave chamber and most likely was considerably deeper in the past.
http://www.wondermondo.com/Countries/NA/US/Florida/CherokeeSink.htm