The Highest Point on the U.S. Interstate System Is...Underground?

The longest mountain tunnel in the world is also the highest point on the U.S. Interstate System -- but its exact location may surprise you.

Last month, driving from the Pacific Northwest down to California, I passed a highway sign in southern Oregon telling me that I'd reached the top of Siskiyou Summit, which is, the sign attested, the highest point on Interstate 5, at 4,310 feet. Think of that! I was at the highest point on this whole road, one that runs from Canada all the way to Mexico. I was grateful to the geography nerds at the Department of Transportation who thought I needed to know this, but I was also suddenly curious: what's the highest point on the entire interstate system? The answer surprised me: it's much, much higher than Siskiyou Summit -- and it's underground.

The interstate system is a modern wonder of the world.

The U.S. interstate system is the greatest peacetime public works project in history. More than 41,000 miles of highway now span one of the world's largest countries, covering a total land area the size of Delaware. The project was authorized in 1956 by President Dwight D. Eisenhower, who was impressed by the well-engineered autobahns he'd seen in Germany during World War II. Many sections of the interstate system were finished in the 1950s and 1960s, but one of the most difficult projects, named for Eisenhower himself, wasn't finished until 1979.

The tunnel burrows right under the top of the Rockies.Loveland Pass is a twisty, often treacherous road across the Continental Divide an hour west of Denver, Colorado. Despairing of building an interstate highway along the pass's hairpin turns, engineers decide to tunnel under the Continental Divide instead. The Eisenhower Tunnel now bypasses Loveland Pass at a height of 11,158 feet, almost three times the altitude of lowly Siskiyou Summit.

Building the Eisenhower Tunnel was a matter of life and death.

At the time of its completion, the Eisenhower Tunnel was the highest tunnel in the world, though it's now been outclassed by the Fenguoshan Suidao, a 16,000-foot tunnel in Tibet. But it's still the longest mountain tunnel in the world, at almost 1.7 miles in length. The high-altitude conditions at the Continental Divide made building a tunnel that long an engineering nightmare, and nine workers died in cave-ins before it was completed.

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