Tower Subway

The Tower Subway is the world first true tube railway, constructed as a narrow iron cylinder just seven feet in diameter. It represents a major leap in engineering, as it was the first to use a cast iron circular shield to bore through the deep London clay beneath the Thames. When construction began in eighteen sixty-nine, the project was finished in less than a year, a staggering pace compared to earlier, brick-lined tunnels that took nearly two decades to complete.