The Gotthard Base Tunnel runs 57 kilometers, and is located about 2 kilometers beneath the Swiss Alps.
"It will stand alone on the engineering front as well. Where its nearest rivals both pass beneath relatively shallow bodies of water, the Gotthard cuts through the complex basement rocks of a giant, heavily folded mountain range. No one has ever tunneled so deep into a mountain, or to such transforming effect.
When it opens, in 2017, it will render Switzerland, for railroading purposes, as flat as Holland. High-speed passenger trains southbound from Zurich will race along a nearly level course all the way to Milan, booming through the Swiss countryside as fast as 250 kmh (155 mph), flashing in one side of the mountains and popping out the other a few minutes later. It will be as though the Alps didn't exist. Travel time between the cities will drop from nearly four hours to just over two and a half-quicker and more direct than if you flew."