Microbes are all around us, and their amounts and identities in a certain area are driven by the available resources. Built environments like houses, office buildings, and transit stations typically harbor fewer nutrient resources than natural ecosystems, but how these low-nutrient conditions affect airborne viruses has remained unclear. To learn more, researchers launched a comprehensive investigation of viruses in the air of public transit systems in 6 cities around the world: New York City, Denver, Hong Kong, London, Oslo, and Stockholm. Using DNA sequencing techniques, the team discovered clear variation in airborne viral communities across the different sites. All these communities were surprisingly diverse given the lack of nutrients, with North American sites showing the lowest diversity overall. Further analysis showed almost half of the potential species were associated with Caulimoviridae, a viral family commonly found in air, while almost one-third could not be classified to a particular group. The uncovered viruses also presented diverse functions, some harboring antimicrobial resistance genes and most showing evidence of a virulent lifestyle. These findings provide new insight into the unique factors driving viral community dynamics in built environments and highlight the need for further investigations to learn more about the large number of unclassified viruses.