Officials say that the fire at an abandoned grain elevator was started when contractors removing wood from the structure cut through a nail, causing a spark that set it alight.
The three workers at the site when the fire started, as well as the elevator's owner, were all accounted for after the blaze started around 3:30 p.m.
The Superior Telegram is reporting that an estimated $10 million-worth of wood reclamation equipment was at the site, which is likely destroyed by the blaze.
The fire is expected to burn for several more days, with a collapse zone put in place around the structure.
However, Mayor Jim Paine issued an update just after 7 p.m. that said the fire is now under control, with the Superior Fire Department extracting water directly from Lake Superior to attack it.
In an earlier update Paine described the former Globe Elevator as being made from "untreated, old growth timber, not much different than what you would throw in a fireplace."
Footage broadcast by ABC affiliate WDIO on Facebook showed the elevator well alight on the banks of the Twin Ports.
The former Globe Elevator, which is abandoned, is at 2nd Street and Banks Ave.
Another angle of the fire can also be seen live on Duluth Harbor Cam.
Superior was of course the scene of another major fire this year, when a blaze broke out following an explosion at the Husky oil refinery in April, injuring 13 people.