The Great Lakes consist of Superior, Michigan, Huron, Erie, and Ontario, and are the world's greatest concentration, more than 20 percent, of fresh water. In geologic terms they are only about ten thousand years old, formed during the last ice age. Appreciating their history has taken on additional importance as foreign marine life has threatened their unique fishing resources, and pollution has intruded on their purity. The National Museum of the Great Lakes in Toledo, Ohio, presents a comprehensive view of the region. The Great Lakes Shipwreck Historical Society maintains historical sites and has identified more than 8,000 shipwreck sites in the lakes, a unique collection of historical evidence through the centuries. Sara Stafford (1853-1927) was active in the Thunder Bay Historical Society and also wrote The Keeper of the Gate, or the Sleeping Giant of Lake Superior, and Port Arthur in Ye Olden Time, in which she related that in the winter the mail came by dog sled over the ice from Duluth and that "We considered that the news was up to date and not old if the papers received were not more than twenty days old."