Red Snapper Sink is located on the continental shelf, about 42 km (26 ml) east of Crescent Beach, Florida. In 1991, advanced technical-diving techniques enabled divers to explore the bottom of the sinkhole for the first time. The opening of the sinkhole at a depth of 27.5 m (90 ft) is approximately 122 m (400 ft) in diameter. The maximum recorded depth in the sinkhole is 147 m (482 ft). The sloping sides of the sinkhole are developed on loose Holocene sand and shell from 27 to 34.5 m (88 to 113 ft). Pleistocene sand and clayey sand crop out from 34.5 to 41 m (113 to 134 ft). From 41 m (134 ft) to about 116 m (380 ft), Red Snapper Sink is a vertical shaft measuring about 45.5 m (150 ft) in width to 52 m (170 ft) in length. The walls of the shaft from 41 to 50 m (134 to 164 ft) transect slightly indurated Pliocene shelly sand. The walls are composed of moderately indurated Pliocene silty sands and sandy silts from 50 to 63 m (164 to 206 ft). From 63 to 102 m (206 to 335 ft), the walls are developed in clayey sands and sandy clays of the Upper Hawthorn Group (Miocene). The Lower Hawthorn consists of a dolomitic limestone containing phosphate pebbles and carbonate interclasts with phosphatic rims from 102 to 116 m (335 to 380 ft). The top of the Ocala Limestone occurs at 116 m (380 ft), and below this depth, the walls of the shaft are undercut. Water samples collected at the bottom show normal seawater specific conductance and chloride and sulfate concentrations. During a dive to 147 m (482 ft), sea water was observed flowing into small caves at the base of the wall, indicating that during the dive period, the sinkhole conveyed saltwater into the Floridan aquifer system. Seismic profiles show that Red Snapper Sink is the surficial expression of a dissolution collapse feature that possibly originated in Upper Cretaceous or Paleocene rocks. Similar buried features in northeastern Florida could provide a hydraulic connection between freshwater zones and deeper, more saline zones of the Floridan aquifer system. The presence of these collapse features could help explain the anomalous distribution of elevated chloride concentrations in parts of eastern Duval County.