Overview: Unearth the technological leaps forward that have allowed the world's longest tunnel - the Gotthard Base Tunnel - to be built. The Gotthard Base Tunnel stretches 57 kilometres through the heart of the Swiss Alps. It is deeper than any other tunnel on Earth and paves the way for a vital high-speed rail link between Zurich and Milan. 2000 workers and four gigantic tunnel boring machines work flat-out to excavate 24 million tonnes of mountain rock to complete the tunnel. Explore how this structure was made possible through a series of six engineering breakthroughs. See how six landmark tunnels - including the Thames Tunnel and Channel Tunnel - each feature an ingenious technological innovation that have enabled tunnels to grow ever longer.