Back in 2007 the History channel started a show called "Cities of the Underworld." It ran a few seasons. A guy went around the world showing all the tunnel systems in major cities, and once under a castle (Romania?). It was an interesting program. There are tunnels everywhere. San Francisco was featured, as well as Portland, OR. It's not just New York and L.A. London, Paris, D.C. and many others were on that show.