It took 20 years and $12 billion to build the tunnel. One of the biggest challenges was the high pressure on the rocks. The high pressure is caused by the weight of the mountain above pushing down on the tunnel, says Kalman Kovari, a professor of tunneling at the Federal Institute of Technology in Zurich, Switzerland, who worked on the project. “It’s what you would experience at 5,000 meters below the sea,” he says.
But the engineers completed the tunnel on time and on budget. “Many people had doubts we could finish the tunnel,” says Kovari, “but we did it.”