For decades, New York sidewalks were awash in piled-up black plastic rubbish bags, some of which doubled as housing projects for the city’s other inhabitants. “I’ve kicked bags of garbage in New York as part of a rat safari and you just watch rats go flying,” says Kaylee Byers, an expert on urban rats at the University of British Columbia. “A rat runs across your foot. You think about it every time you’re on the block for the rest of your life,” says Joshua Goodman, a deputy commissioner of sanitation in New York. A study conducted in 2023 by MMPC, a pest-control company, suggested there were 3m rats in the city, up from 2m a decade earlier.