Compared to the 160 million years the Alps have spent climbing out of the violent collision of the Eurasian and African tectonic plates, Switzerland's 17-year project to blast through them doesn't seem lengthy at all. But the feat fits the scale of the snow-capped mountains: When it opens today, the Gotthard Base Tunnel will be the longest, deepest rail tunnel in the world, spanning 35 miles and dipping to 1.4 miles below the surface. It's a majorly challenging and ambitious infrastructure project for a small country perhaps best known for chocolate. Here's what you need to know.

It's about making mountain journeys less of a pain.

The Gotthard runs the 35 miles from the picturesque town of Erstfeld to the picturesque town of Bodio. It's actually the third tunnel in the area, joining the Gotthard road tunnel, which carries automobiles, and the Gotthard rail tunnel, a much shorter train passage opened in 1882. The new setups cost about $12.3 billion, funded in part by a new road tax passed by Swiss referendum in 1998. In December, high-speed trains will start rocketing between the two, replacing an hour-long drive with a 20-minute jaunt. By linking with existing rail networks, the new tunnel will also halve  the more popular, 4.5-hour journey from Zurich to Milan.

Credit: AlpTransit Gotthard Ltd.

Building it was a pain.

Over 17 years of construction, tunnel boring machines—named Heidi, Sissi, Gabi, and Gabi II—bore the brunt of the work. Over 2,400 workers toiled round the clock in three shifts to excavate over 30 million tons of granite, gneiss, and sedimentary rock. During the climax of the work, underneath the humongous Gotthard Massif range, temperatures in the tunnel reached a distinctly un-Swiss 122 degrees Fahrenheit. In total, the construction team built 94 miles of tunnel, including two side-by-side shafts and a network of access shafts, emergency passages, and air ducts to support them.

Credit: AlpTransit Gotthard Ltd.

The tunnel will protect the Alps.

These days, the hills are mostly alive with the sound of traffic. The vehicle-friendly Gotthard road tunnel alone plays host to five million cars and 900,000 heavy trucks a year. Congestion is bad enough, but idling vehicles—and especially trucks—mean the cool mountain air gets its fair dose of pollution: France's Alpine towns are some of its most polluted, according to the World Health Organization report. So the new rail tunnel's builders are hoping it can attract people and goods away from the Alps's roads and open air. Maria Von Trapp would approve.