Deep Lake, the 90ft deep sink hole in Florida. Freshwater on top and Saltwater on the bottom.

 

Found on the far western end of the Preserve, there exists a unique feature in the state of Florida, with an equally unique history. This 90-foot-deep naturally occurring sinkhole lake is the deepest lake south of Lake Okeechobee, and one of the deepest in the entire state. Interestingly, studies show the top half is freshwater, and the lower is saltwater.Because of its abundance of lush vegetation, water, and high ground, it is a magnet to not just wildlife, but for people as well. 

 Shortly after 1900 the Deep Lake area was acquired by Walter Langford and John Roach, with plans to develop the land into a grapefruit enterprise. 200 acres were cleared and planted with Marsh seedless grapefruit trees and a railroad tram system was built, with a small four wheeled rail car and flatbeds to transport their crop output. The line eventually extended to Everglades City, and by 1915 17,000 boxes of Deep Lake grapefruit were being brought to market. In 1922 Barron Collier purchased Deep Lake and built a large cottage home there. The Atlantic Coastline Railroad bought the line from Deep Lake to Everglades City, revamping it become part of the extensive ACL route in 1928. A prison was built across State Road 29 from the grapefruit groves as well. The rail line from Immokalee thru Deep Lake to Everglades City was abandoned in the 1960's, and the prison was taken over by the Dept of Transportation before being abandoned in 2002. Today the Deep Lake area is part of the vast Fakahatchee Strand Preserve State Forest.

Sources https://www.nps.gov/bicy/learn/historyculture/deep-lake.htm

 http://www.ghosttowns.com/states/fl/deeplake.html