Chicago was home to an incredible number of amazing engineering feats in the mid- to late-1800s.
The city was settled on low-lying flat ground at the shores of Lake Michigan — a swamp, essentially — so as a result muddy streets and unsanitary conditions quickly became a huge problem, and a cholera outbreak in 1845 killed 6% of the population.
Clearly the city could not grow further without major improvements.
They needed a sewer system to handle draining away both sewage and surface water — but in a swamp, gravity is your enemy, not your friend.
The sewer system had to be above the current streets. So they raised the streets, the buildings, all of it. Over about 20 years, starting in 1858.
They raised an entire downtown city block, several feet, all at once, in four days, while people were conducting business inside the buildings. In the 1860s.
Earlier post about the large underground tunnel network, largely forgotten until 1992 when the river above started draining into it and flooded many downtown buildings.