A few days ago, our group stayed in Germany in Rothenburg ob der Tauber (Rothenburg on the Tauber river). We stayed in a hotel built from a restored flour mill, and the owner showed us around the inside of it!
The mill itself was built sometime in the 1500's or 1600's. Over time, it fell into disrepair, but it was restored in the 1900's with new assembly-line parts. Recently, the owner updated the mill yet again, and now instead of grinding flour the mill creates hydroelectric power to help support the hotel.
Today, I went down into the Paris sewers. They are some of the If you straightened out the Paris sewer system, it would reach from Paris to Istanbul.
Before the sewers, the people of Paris in the middle ages just used the Seine, and over time different developments to the sewers were made. Napoleon, for instance, ordered a huge addition to the construction of the sewers. A cholera outbreak afterwards, though, lead to the massive redesign in 1850 by Eugène Belgrand.