The New York Times has a great article about the huge effort that will be required to rebuild the subway tunnels underneath the World Trade Center site. More than a mile of tunnels need to be rebuilt, at a potential cost of $1 billion or more. Complicating things is the fact that the city is going to have to fill part of the tunnel system with concrete in order to short up the roads above for recovery equipment; construction crews will then have to wreck all that concrete out of the tunnels when the time comes.