The steamship Illinois became the first ship to pass through the Soo Locks, connecting Lakes Huron and Superior via the St. Mary's River at Sault Ste. Marie. Laborers worked two years building two 350-foot locks, arranged in tandem to overcome the rapids caused by the twenty-one-foot difference between the lakes. The state of Michigan operated the locks until 1881 when they were transferred to the control of the federal government. Today, the locks remain one of the world's busiest navigation routes.