The Busiest Lock You've Never Heard Of

Nearly all of America’s domestic iron ore — the stuff that becomes the steel in cars, appliances and buildings — floats through one set of locks in Michigan, and ships pay nothing to use them.

Up in Sault Ste. Marie (“the Soo”), Lake Superior sits about 21 feet higher than the lakes below it, along the St. Marys River. Ships can’t sail down a waterfall, so since 1855 the Soo Locks have gently raised and lowered freighters between the lakes — using nothing but gravity and water, no pumps. The U.S. Army Corps of Engineers operates them and lets vessels pass free of charge.

The scale is staggering. The Corps of Engineers reports that more than 4,500 vessels carrying up to 80 million tons of cargo move through the Soo each year, making it one of the busiest lock systems in the world by tonnage. The Corps also states that “nearly 100% of America’s domestic iron ore passes through the Soo Locks” — which is why they’re sometimes called the “linchpin of the Great Lakes.” During World War II, the region was among the most heavily defended places in North America, because the steel for Allied tanks, ships and planes depended on it.

Right now a massive new lock — the same size as the big Poe Lock — is under construction, on track for completion in summer 2030 at a cost recently estimated around $2.6 billion, to add a much-needed backup for the giant 1,000-foot freighters.