These 5 Very Long Tunnels Are Engineering Marvels

Tunnels are a form of infrastructure most people spend little time thinking about. Out of sight really does take something out of mind. But humans have been digging tunnels to move critical materials from one place to another for millennia. While the 3,400-foot Tunnel of Eupalinos, an early aqueduct, dates to the 6th century B.C., archaeologists believe the oldest irrigation tunnels (or qanats) were dug around 4,000 years ago, and temple of Belos may have been connected via tunnel to the palace in Babylon in 2,200 B.C.

Today, tunnel networks spread like webs below most major cities, separating highways and transit lines from traffic on the surface or bringing in water and taking away waste. While most tunnels are relatively short, the most impressive ones can extend for miles. Below, we list some of the longest tunnels based on what they help transport.
 

Lærdal Tunnel, Norway

Road tunnels are popular in mountainous regions, as they can help shorten a trip that would otherwise need to climb up to a pass—and avoid closure during winter weather. They are also perfect for connecting islands to the mainland when bridges would be impractical. The Lærdal Tunnel is part of a highway connecting Norway’s two major cities—Oslo and Bergen—and avoids both a mountain pass and a ferry trip. At more than 15 miles, the tunnel takes motorists some 20 minutes to traverse.

To break up such a long, underground drive, the engineers who built the Lærdal Tunnel included what they called “caves”: large, brightly lit caverns where drivers can park or turn around. There are even reports of couples holding their weddings in the caves.
 

Large Hadron Collider, Switzerland and France

While tunnels are typically used to move things from point A to point B, the cargo in the circular passageway of the Large Hadron Collider goes around and around and often never gets out. When the 17-mile circumferential tunnel was first constructed between 1983 and 1988 for an earlier physics experiment, it was the largest construction project in Europe. The facility was closed in 2000 and revamped to create the world’s largest particle accelerator. 

The tunnel itself is 12 feet wide and varies between 160 and more than 500 feet below the surface. (The tunnel is so big it crosses the border between Switzerland and France.) Magnets line the interior to direct two beams of ions traveling at nearly the speed of light in opposite directions; it takes only 90 microseconds for the particles to make a complete circuit. Where the beams intersect, the collisions between ions produce a shower of subatomic particles which scientists study to understand the fundamentals of physics. 
 

Gotthard Base Tunnel, Switzerland

While the Large Hadron Collider tunnel is a large loop, the Gotthard Base Tunnel is a different part of Switzerland is a straight shot through the Alps. Excavated between 1999 and 2016 (and requiring the removal of 17,000,000 cubic yards of rock), the tunnel replaced a shorter railway tunnel dating to the 1800s that was approached via a series of loops and switchbacks. The 35-mile tunnel created a shortcut that saves approximately an hour travel time on trains running between Zurich and Milan.  

Passenger trains typically run at 125 miles per hour through the tunnel, with a theoretical top speed of 155 mph, so a trip through the tunnel takes about 20 minutes. Conditions can be markedly different on the other side: The Alps are such a barrier and the tunnel is so long that the weather differs greatly on each end. On average, temperatures differ by 5 °F between the ends.
  

Neelum–Jhelum Hydropower Plant, Pakistan-administered Kashmir

Tunnels can form a critical piece of hydroelectric power stations, as they can efficiently transport water from a high reservoir to a set of turbines at a much lower altitude while preserving the power-producing hydraulic head. Opened in 2018, the tunnel built at the Neelum–Jhelum Dam on the Neelum River carries water more than 40 miles to a power station on the Jhelum River that’s 1,378 feet lower. In fact, the tunnel is so long and the terrain is so twisty that at one point the tunnel runs underneath the Jhelum River itself. 

Under normal operating conditions, the turbines produce 969 MW of electricity. However, in 2024 water pressure reaching the turbines dropped significantly and the power production was halted to find the cause. One potential problem may be the collapse of the tunnel somewhere along its 40-mile length. Finding the blockage has been a challenge, and at present, the plant is still offline.
 

Delaware Aqueduct, United States 

Considering that one of the first uses for tunnels was to transport water, it’s fitting that the longest tunnel in the world is an aqueduct to carry drinking water to New York City. The Delaware Aqueduct is 13.5 feet wide, 85 miles long, and delivers about half a billion gallons of water per day.

Built between 1939 and 1945, the aqueduct connects the city to a 1,000 square mile watershed of rivers and reservoirs in the Catskills Mountains. Because the city is built on land surrounded by salt water (even the Hudson River is salty at New York), its 8 million residents are reliant on water brought in from more than 100 miles away. That has its advantages, too, since the water doesn’t contain dissolved minerals commonly found in groundwater or water from larger rivers. 

The water reaching the city from this and other aqueducts is distributed via a series of vast tunnels running deep under the city streets. The newest tunnel, New York City Water Tunnel No. 3, will be 60 miles long when it is completed sometime in the early 2030s.

Jeffrey Winters is editor in chief of Mechanical Engineering magazine.