In the early 1900s a few of the city planners in Chicago thought it would be advantageous to connect North Michigan Avenue with South Michigan Avenue via a bridge across the Chicago River. It took three years (1917-20) to complete the project, but they didn’t simply create a bridge, they created an architectural masterpiece.
Evidently this bridge is an engineering wonder, but my reference on the Michigan Avenue Bridge described it in engineering terminology and I cannot begin to explain why it is such a feat. I did come away with one fact I find impressive though. Evidently the bridge, a double-decker affair, is so well balanced it only takes two 108-horsepower motors to open and close each 3,750-ton extension. That is efficient! I motor across the lake in a fishing boat that has a bigger engine!