| In the second half of the 19th century, Chicago was one of the fastest-growing cities in the world. In 1870, it was home to 299,000 people, and by the century's end, 1.7 million. But along with that population boom came unfortunate side effects, including waterborne diseases such as cholera and typhoid. The problem was in large part that the city's sewage flowed into the Chicago River, which in turn emptied into Lake Michigan — the source of the city's drinking water. So Chicago turned to engineer Ellis S. Chesbrough, designer of the city's sewer system, to solve the problem once and for all. Initially, Chesbrough designed a 2-mile-long tunnel 60 feet below the bottom of Lake Michigan to draw less-polluted water from farther offshore. Unfortunately, all it took was a heavy rain for this far-flung water source to also become polluted, so Chesbrough eyed another solution. If the city's eponymous river could just flow away from Lake Michigan and empty into the waterways leading to the Mississippi, Chicago's water problem would be solved. The subcontinental divide just west of Chicago is what caused the river to flow toward the lake, so if the city dug a ditch lower than both the lake and the river through the divide, gravity would take it from there. Workers began the laborious process of reversing the Chicago River in 1892. After eight years of digging (and under cover of night due to mounting lawsuits from cities downstream), Chicago blew up the last dam on January 2, 1900. Chesbrough never saw the incredible feat of human engineering — he died in 1886 — but his ambitious plan saved the city, securing its prosperous future into the 20th century and beyond. | 
