Underneath the streets of London, a team of more than ten thousand construction workers race to build a brand new metro line—Crossrail. Costing almost $23 billion, it's the biggest engineering project in Europe and must link into the existing metro system. As they burrow the 26 miles of tunnels, engineers battle to make sure historic buildings don't crack, London Underground trains keep running, and an ambitious station roof made up of 2500 pieces comes together on time. Crucially, they must drive one of their gigantic 1000-ton tunnel boring machines through the earth, passing within inches of escalators and an active subway tunnel, without the passengers on the tube platforms below ever knowing they are there. Join NOVA to plunge into the tunnels of the London Underground and follow this high stakes, action packed engineering endeavor, and discover just how engineers are performing this delicate surgery through the heart of the historic city.