The Warm Mineral Springs is a water-filled sinkhole located in North Port, Florida, a mile north of U.S. 41. The primary water supply is a spring vent deep beneath the pool's water surface. Warm Mineral Springs is the only warm water mineral spring in the State of Florida. It is an important geological and archaeological site containing Native American remnants. The site was operated as a spa from 1946 until 2000. It was added to the U.S. National Register of Historic Places on November 28, 1977. The springs re-opened for swimming only in 2014.
Geology
Warm Mineral Springs is a sinkhole formed in carbonate rock by the collapse of the roof of a cavern 30,000 years ago. The land surrounding the sinkhole is flat, and about above mean sea level. The circular opening of the sinkhole at the current water level is across. The sinkhole is deep, and is shaped roughly like an hourglass. The opening narrows to across a few meters below the surface of the water. About below the surface the opening widens for a short space, forming a ledge under an overhang. The opening narrows to across at below the surface, and then opens up again, reaching across at the bottom of the sinkhole. Debris from the collapsed roof and sides of the sink, and other material that has fallen into the sink, forms a large cone at the bottom, rising to below the surface of the water.
The environment in Florida during the Pleistocene period was much different than today. Due to the enormous amount of water frozen in ice sheets during the last glacial period, the sea level was at least lower than it is today. Florida had about twice the land area of today, its water table was much lower, and its climate was cooler and much drier. There were few running rivers or springs in the part of Florida that is now above sea level. The few water sources in the interior of Florida were rain-fed lakes and water holes perched on relatively impervious deposits of marl and deep sinkholes partially filled by springs, such as Warm Mineral Springs. The water level in the sinkhole 12,000 years ago was about lower than it is today. Dripstone (stalactites and stalagmites) has formed on the walls of Warm Mineral Springs down to about below the present level of the water, indicating that the water level in the sinkhole was at least low long enough for the dripstone to form.
Although there are many small freshwater springs in the walls of the sinkhole, the primary source is a vent below the surface of the water which discharges an estimated a day of warm [] mineralized water. The water has a high content of dissolved chloride (1,600 to 1,700 milligrams per liter [mg/L]) and hydrogen sulfide (9,200 to 9,600 mg/L), and essentially no dissolved oxygen. The source of the water is uncertain. The water enters the sinkhole at the level from a conduit that extends with several turns to a point from the entrance and below the surface of the water. The conduit has an elliptical shape, about high by wide, for much of its length. At the far end the conduit opens up into two small caverns. Most of the flow appears to enter through the floor of the last cavern. Between and a day overflows from the sinkhole.
History
The area surrounding the spring was developed as a spa in 1946, including the construction of a bathhouse. The property was sold in 1955 and the new owners began a housing development, platting 3,000 housing lots on the property. The Warm Mineral Springs Motel was opened at the entrance to the spring in 1958. In 1959 a Park Spa Building and Cyclorama (depicting Ponce de Leon's alleged discovery of the Fountain of Youth) were built at the spring to house a Florida Quadricentenial celebration, which ran from December 14, 1959 to March 15, 1960. The Spa Building and the Cyclorama were designed by Jack West, a leader of the Sarasota School of Architecture. The Spa Building, the Cyclorama, and a sales building attached to the Spa, were added to the National List of Historic Places in 2019 as the Warm Mineral Springs Building Complex.
In the late 1950s William Royal and other scuba divers found artifacts and human bones from at least seven individuals in Warm Mineral Springs. A partially burned log found in association with some of the human bones was radiocarbon dated to about 10,000 years ago. If the bones were the same age as the log, then the bones were the oldest known evidence of human occupation in Florida at the time. Bone artifacts recovered in association with the human bones were described as of "Archaic period in the Americas-type." Apparently, the bones and artifacts were deposited above the water level in the sinkhole at that time.