The Great Hanoi Rat Massacre of 1902 Did Not Go as Planned

At the turn of the 20th century, Vietnam was a French colony. The administrators built their own French neighborhood in Hanoi to house their families, complete with a modern sewer system underneath. That particular amenity of Western life was not compatible with the existing ecosystem of Vietnam.

It turns out that when Doumer’s colonial government laid more than nine miles of sewage pipe beneath Hanoi, it inadvertently created nine miles of cool, dark rodent paradise, where the pests could breed without fear of predators. And when they got hungry, the rats had direct access to the city’s ritziest real estate via a subterranean superhighway. Under the streets of French Hanoi, rats multiplied exponentially—and then skittered to the surface.
As if it wasn’t enough that these furry invaders disrupted the colonists’ illusion of European tranquility in Asia, cases of the bubonic plague started popping up, and rats were suspected of carrying the disease. Something had to be done.

The colonial government waged a war on rats that began with professional rat hunters, but soon expanded to offering a bounty to the public: a penny for each rat killed. You might be able to see where this is going, and you'd be right. No matter how many bounties were paid, the rats only multiplied. Read the story of the Great Hanoi Rat Massacre at Atlas Obscura.