The Soo Locks are a series of locks located on the St. Marys River between Lake Superior and Lake Huron in the United States. They allow ships to navigate the change in water levels between the two lakes, which can be as much as 21 feet. The locks are essential for shipping, enabling the transport of goods like iron ore and grain. Constructed in the mid-19th century, the Soo Locks are one of the busiest lock systems in the world. They facilitate the movement of over 10,000 vessels each year, making them a crucial part of the Great Lakes shipping industry.