transmissivity tends to be relatively high. Where the aquifer is thickly confined, less dissolution occurs and transmissivity tends to be lower. In the first regional map depicting transmissivity variation across the aquifer, Miller (1986) showed that transmissivity values exceed 250,000 ft2/d () where the aquifer system is either unconfined or thinly confined. In areas where the aquifer is thickly confined, Miller (1986) indicated lower transmissivity was related primarily to textural changes and secondarily to the thickness of the rocks. Micritic limestone in southern Florida and in the updip outcrop areas was identified as having much lower transmissivity than elsewhere in the system. Sinkholes Sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by groundwater circulating through them. As the rock dissolves, spaces and caverns develop underground. If there is not enough support for the land above the spaces then a sudden collapse of the land surface can occur. These collapses can be small or they can be huge and can occur where a house or road is on top. Sinkholes can be classified on the basis of the processes by which they are formed: dissolution, cover-subsidence, and cover-collapse. Formation of sinkholes can be accelerated by intense withdrawals of groundwater over short periods of time, such as those caused by pumping for frost-protection of winter crops in west-central Florida. Sinkholes that developed beneath gypsum stacks in 1994 and 2016 caused a loss of millions of gallons mineralized water containing phosphogypsum and phosphoric acid, by-products of the production of fertilizer from phosphate rock. These sinkholes were likely formed from the collapse of preexisting dissolution cavities in the limestone beneath the stacks. Lake Jackson near Tallahassee, FL occasionally drains into a sinkhole in the bottom of the lake bed when water levels in the aquifer drop. Dover Sinkhole, located along the Peace River near Bartow, FL, was witnessed draining about 10 Mgal/d () of water from the Peace River during June 2006. Springs There are 824 springs inventoried across the Floridan aquifer system of which 751 are located in Florida, 17 in Alabama, and 56 in Georgia. Springs are classified according to median value of all available discharge measurements. In Florida, there are 33 magnitude 1 springs, the more notable of which include: Spring Creek (Wakulla County) Three Sisters Springs (Citrus County) Silver Springs (Marion County) Rainbow Springs (Marion