It was the dawn of 1863, and London's not-yet-opened subway system — the first of its kind in the world — had the city in an uproar. Most people thought the project, which cost more than 100 million dollars in today's money, would never work. So how did they do it? Christian Wolmar explains how the London Underground was built at a time when no one had built a railway under a city before. Lesson by Christian Wolmar, animation by TED-Ed.