More than 10 million acres burned across the country during the 2017 U.S. wildfire season at a cost of more than $2 billion — the largest bill ever. And while many factors affect the risk for wildfires, new research out of the University of Wisconsin–Madison shows that a flurry of homebuilding near wild areas since 1990 has greatly increased the number of homes at risk from wildfires while increasing the costs associated with fighting those fires in increasingly dense developments. The so-called wildland-urban interface, or WUI, where homes and wild vegetation meet, increased rapidly from 1990 to 2010, adding a collective area larger than the state of Washington across the contiguous United States.