Well, I know that this is a little underground theme, but sewers are one of the most complexest and important structure in every city. Without that the living would be much bad smelling and even worse.
It is 100 years when Prague as a one of the first cities in Europe started to clean a sewage and therefore stopped bad smell in streets and in a river. Dozens of kilometers of sewers was made and the Prague sewage clarification plant started its work hundred years ago. The steam engines worked 20 years without stop. In 1967 the plant ended its work and the new one started at another place.
Today the canalization has 3,5 thousand kilometers. If is for you 100 years too small piece of history than let me tell you that oldest canalization in Prague is from 12th century and it was built for Strahov monastery.