Lake Erie is a favorite for divers since there are many shipwrecks, perhaps 1,400 to 8,000 according to one estimate, of which about 270 are confirmed shipwreck locations. Research into shipwrecks has been organized by the Peachman Lake Erie Shipwreck Research Center, located on the grounds of the Great Lakes Historical Society. Most wrecks are undiscovered but believed to be well preserved and at most below the water surface. One report suggests there are more wrecks per square mile than any other freshwater location, including wrecks from Native American watercraft. There are efforts to identify shipwreck sites and survey the lake floor to map the location of underwater sites, possibly for further study or exploration. While the lake is relatively warmer than the other Great Lakes, there is a thermocline, meaning that as a diver descends, the water temperature drops about , requiring a wetsuit. One estimate is that Lake Erie has a quarter of all 8,000 estimated shipwrecks in the Great Lakes. They are preserved because the water is cold and salt-free. Divers have a policy of not removing or touching anything at the wreck. The cold conditions make diving difficult, requiring divers with skill and experience. One charter firm from western New York State takes about 1,500 divers to Lake Erie shipwrecks in a typical season from April through October.