(CNN) — The world's longest, deepest tunnel -- running for 57 kilometers underneath the Swiss Alps -- has finally entered regular service.
The Gotthard Base Tunnel, dubbed Switzerland's "construction of the century," was 17 years in the making and is reported to have cost around $12 billion.
It began operating a full service on December 11, 2016. To mark the occasion, we're taking a look at 10 of the world's greatest tunnels.
1. Gotthard Base Tunnel (Switzerland)
The world's longest and deepest tunnel has been inaugurated. The company behind it tells CNN how construction was made possible.
European leaders including German Chancellor Angela Merkel and French President Francois Hollande turned out for the Gotthard's Base Tunnel's grand opening in June, which featured colorful and often surreal scenes involving costumed dancers, fireworks and plenty of yodeling.