The Great Lakes, so easily taken for granted by North Americans, are a unique and awesome feature on the planet. These "sweetwater seas," as early explorers described them, stretch for over 1,000 miles and hold one-fifth of the Earth’s supply of fresh water. They are the heart of a region of forests, thousands of small lakes, and praries that once teemed with wildlife. But with all of that water, timber and rich soil, European settlers transformed the Great Lakes domain of fur trappers and Native Americans into a frontier of farming and industry after the Erie Canal opened up transportation from the east Coast in 1825. The great "heartland" cities of Chicago, Cleveland, Toronto, Milwaukee, Detriot, Toldeo, Buffalo and other sprang ip rapidly. Although the northern perimeter of the Great Lakes still remains relatively wild and forested, 35 million people live around the lakes, mostly in its lower reaches, producing goods and services that amount to roughly one-fifth of the United States economy.