The invaders are already among us — just check your local subway. Cities all over the world are seeing a serious spike in rats, with tourism and public health suffering as a result. The real problem? Surprisingly few extermination methods, high-tech or otherwise, work in major urban areas. That’s not to say people are short of ideas. Somerville, Massachusetts, encourages citizens to call 311 whenever they see a rat, and a recent Roman mayoral candidate suggested enlisting half a million feral cats to get the job done.