The Gotthard Base Tunnel runs 57 kilometers beneath the Swiss Alps from the canton of Uri to Ticino, the longest railway tunnel and deepest traffic tunnel in the world, with up to 2.5 kilometers of rock overhead that would bake at 46 degrees Celsius without ventilation. The Gotthard Pass has linked the Rhine and the Po since the 13th century, but in the 1840s a five horse stagecoach took 23 hours to cross, and the 1882 railway needed spiral tunnels to climb. After the 1980 road tunnel brought a tenfold rise in trucks, Swiss voters passed the 1994 Alps Initiative, and a 2000 deal with the EU raised truck limits to 40 tons while a distance and weight fee on heavy trucks helped pay for the 12.2 billion franc tunnel.

Work ran from 1999 to 2016 from up to five sites, including Amsteg, Sedrun, Faido, Erstfeld, and Bodio, removing 28.2 million tons of rock. Four 440 meter boring machines named Sissi, Heidi, Gabi 1, and Gabi 2 cut 25 to 30 meters a day, and gyro theodolites corrected for the mountain’s gravity pulling plumb lines sideways, so the tubes met within millimeters. Nine workers died, honored at the June 2016 opening by clergy of four faiths. Zurich to Milan got an hour faster, until August 10, 2023, when a fatigue crack broke a freight wagon wheel, derailing cars at a crossover and wrecking eight kilometers of track until September 2024.

• How orographic lift and the foehn wind can mean snow at one portal and sun 10 degrees warmer 20 minutes later
• Why a flat tunnel at just 549 meters elevation lets one locomotive pull 3,600 tons of freight
• Twin single-track tubes linked every 325 meters and underground multifunction stations for crossovers and evacuation
• A shrine to St. Barbara, patron saint of miners, carved into the tunnel rock
• Measurements showing the mountain surface above the tunnel is subsiding as the Alps adjust to the void