On May 26, 1675, the city of Delft was inundated by a storm. Leeuwenhoek, then forty-two, gathered some of the water from the drains of his rooftop, let it stand for a day, and then put a droplet under one of his microscopes and held it up to the light. He was instantly entranced. No one he knew had seen anything like it. The water was roiling with dozens of kinds of tiny organisms—“animalcules,” he called them.
