In the catacombs, graveyards and sewers, rats feast and breed. Their community is larger than the entire human population, and every year hundreds of these humans decend into such places and disappear, often assumed to have been consumed by the numerous rats. This is an explanation that works, and one that most people at the turn of the 19th century was happy with. But the vanishing people didn't always decend into the sewers voluntarily, or at all.
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