The Great Lakes cover nearly 95,000 square miles (250,000 square kilometers) and hold over 20% of Earth's surface fresh water. More than 30 million people in the U.S. and Canada rely on them for drinking water. The lakes support a multibillion-dollar maritime economy, and the lands around them provided many of the raw materials—timber, coal, iron—that fueled the Midwest's emergence as an industrial heartland.