The Torture Doctor / AP

When a building gets nicknamed the Murder Castle, you know the story behind it is going to be bad. In 1886, H.H. Holmes, a pharmacist who would later be called the first U.S. serial killer, bought a Chicago drugstore that was owned by a cancer-stricken man named E.S. Holton. When Holton died, Holmes bought up surrounding property until he'd acquired an entire city block. He renovated the buildings and turned them into a hotel just in time for the 1893 World's Fair. But this was no ordinary hotel: most of the rooms were windowless, with stairways to nowhere and hallways that ended in dead ends. Holmes also built gas jets into hotel-room walls, a wooden disposal chute and person-size kiln in the basement. This was the perfect place to murder someone. And that's exactly what Holmes did: for much of 1893, he tortured and killed an untold number of people at his hotel, mostly young women visiting the city for the World's Fair. Holmes was caught and eventually hanged. He admitted to killing 27 people, although authorities still wonder if the body count might be dozens more. Strangely, the first floor of the Murder Castle remained a proper drugstore. Every day, customers purchased tonics and medicine, unaware of the horrors taking place directly above them.

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