We used to dig up roads to put trains underneath – cheaply. Ever-better tunnel boring machines have made the disruption this causes unnecessary.Tunneling is one of the many technologies that make modern ​civilization possible. For one, a tunnel can dramatically reduce transportation costs by shortening travel times between two points. Prior to the construction of the Holland Tunnel beneath the Hudson River in New York, for instance, the only way across was via ferry, a journey that could take ho...