The Great Lakes are the largest system of fresh water on earth containing approximately 20% of the world’s fresh water supply. Only the polar ice caps contain more fresh water. The Lakes contain about 5500 cubic miles of water, covering a surface area of 94,000 square miles. Even though the Great Lakes are large, it is very fragile environmentally. Over 20 million people receive their drinking water directly from the Great Lakes. The time for the water in a lake to completely replenish itself runs from 22 years for Lake Huron, to 191 years for Lake Superior. Pollutants in the Great Lakes will remain for many years, so preventing discharge of pollutants is paramount. Compulsory Pilots are an added extra layer of protection that helps insure that pollution from ships does not occur on the Lakes.