A long horn bellow followed by two shorter blasts is the traditional greeting from ships to the bridge tender in Duluth's harbor. The bridge obliges with a similar greeting and rises to let the boat pass into Lake Superior.
With sophisticated GPS systems and radios, these greetings are mostly ceremonial nowadays. The two parties have been greeting each other for over a century as payloads of, among other things, taconite, grain, coal, and sugar make their way in an out of the harbor.
Some of the boats are around 1000 feet long, and their gargantuan metal hulls give off sounds all their own.
You can find a comprehensive list of which boats are docked and when they'll be coming in and out of the harbor at the Duluth Shipping News site.
Ken Newhams runs the Duluth Shipping news. His office practically sits at the foot of the bridge. He's been fascinated with the boats since moving to Duluth 20 years ago, and he's been documenting their schedules for the last 15.
Newhams says that permanent residents barely acknowledge the deafening whistles, but the boats, and their sounds, remain a big draw for tourists.