Overview: New technology is allowing us to build underground structures that far surpass any building on earth. South Dakota is building a mile deep structure, inside an abandoned gold mine, to study the universe. London is building a "silent" rail network to prevent gridlock. Mexico City is burrowing 39 miles underneath the city to save it from drowning. On this episode: Sanford Underground Research Lab; London Cross Rail and Mail Rail; Mexico City Emisor Oriente.