In North America, a small set of vulnerable patches within large power grid networks is disproportionately responsible for costly cascading power failures, a new study finds. These vulnerable components, the authors say, are typically geographically close and are often located near densely populated areas. Sometimes a power failure can be fairly local, but other times, a seemingly identical initial failure cascades to cause a massive and costly breakdown in the system. Understanding the origins and causes of cascades is challenging because the conditions of power grid systems can vary greatly by peak usage times, seasons, and other factors. To better understand how cascades are triggered, Yang Yang et al. analyzed data from the Federal Energy Regulatory Commission, which represented various snapshots of the U.S.-Canadian power grid system, spanning the years 2008 to 2013. Using the data, they developed a model of the connectedness of networks across the continent. Their simulations revealed that, over the entire network, only 10.8% of all links ever underwent a so-called primary failure, which could trigger a cascade. Furthermore, the interconnectedness of a network strongly influenced the likelihood of a cascading power failure. For example, in a network that was not strongly linked to others, almost all links showed zero vulnerability to triggering a cascade, whereas in networks that were more centrally linked, 7 to 19% of links were found to be vulnerable. Larger cascades were associated with concurrent triggering events that were geographically closer to each other and closer to the set of vulnerable components. This study is highlighted in a Perspective by Raissa M. D'Souza.