The Sault Locks (usually calle the Soo Locks) allow ships to travel between Lake Superior and the lower Great Lakes. The locks are the busiest in the world, passing an average of 12,000 ships ("boats" in Great Lakes parlance) per year. This is achieved in spite of the locks being closed during the winter, from January through March, when ice shuts down shipping on the Great Lakes. The winter is used to inspect and maintain the locks.