SAULT STE. MARIE - Winter maintenance season at the Soo Locks offers an entirely different perspective of the hulking transportation system that is the engineering linchpin of Great Lakes shipping.

The locks began its three-month seasonal shutdown on Jan. 15 so crews could address some major repair projects, regular inspections and maintenance.

The U.S. Army Corps of Engineers Detroit District office this week is sharing some pictures of the winter work. Among the most impressive shots are a peek at what the locks look like when they're drained of water that's normally used to raise and lower ships as they're transiting between Lake Superior and the lower water level of Lake Huron.

"It took several 300+ horsepower pumps 16 hours, and by Saturday morning the Poe Lock was nearly empty," the Army Corps said in a photo posted to Facebook on Wednesday.

Of the Soo's four locks, only the Poe and the MacArthur locks are in regular use in the St. Marys River during the shipping season.

Any ships larger than 730 feet long need to go through the Poe, including the handful of 1,000-foot freighters beloved by "laker" watchers in our region.

It takes about 22 million gallons of water to raise the level of the Poe Lock by 21 feet when a big ship comes through, the Army Corps said.

So emptying this vast rectangle and prepping it for repairs is no small feat.

Part of the work being done is a $2.4 million upgrade to the Poe Lock. Gate pieces are being replaced to remove what the corps described as "aging and deteriorating infrastructure."

Ryba Marine Construction of Cheboygan is the contractor for that project.

Each year, more than 4,500 ships move 80 million tons of cargo through the locks, the Army Corps said.

The locks are set to reopen on March 25.