Color postcard from 1920 showing a bird's-eye view of the Soo Locks in Sault Sainte Marie, Michigan. This postcard is from the souvenir postcard folder "Souvenir of Sault Sainte Marie "The Soo" Michigan." The Soo Locks, located on St. Marys River between the United States and Canada, provide ships safe passage around the rapids in the canal. The first locks in this canal were built in 1855, and have been improved upon ever since. The locks are the busiest, based on tonnage passing through, in the world.