1. Laerdal Tunnel
Location /// Sogn og Fjordane, Norway
Background: About 5000 different blasts were conducted during the construction phase of Laerdal Tunnel, which stretches under Norway's mountainous and fjord-filled terrain for 15 miles, making it the longest completed road tunnel in the world.
Why It's Unique: Driving through a windowless tunnel for 20 minutes can get a bit monotonous, so a team of psychologists and engineers focused on retaining driver concentration. "The psychological reaction of a person in a tunnel is very important … it makes the difference between people accepting the facility or simply just avoiding it," says Youssef Hashash, a professor of Civil and Environmental Engineering at the University of Illinois, who has worked on 10 different tunnel projects. "Given the length of this tunnel, you need a carefully designed environment and lighting system." Some solutions used in the Laerdal tunnel include bright blue lights and subtle curves that keep drivers engaged. Most important, though, is the fact that the tunnel is divided into several different sections, breaking up the drive and creating the impression that commuters are traveling through a handful of smaller tunnels.
2. Channel TunnelLocation /// Coquelles, FranceBackground: Commonly known as the Chunnel, this behemoth structure links England and France. Completed in 1994, the price tag of this 31-mile-long tunnel exceeded $20 billion.Why It's Unique: Linking France and England via the English Channel is an idea that has been kicking around for hundreds of years. "There were previous attempts to build a tunnel here, but they were stopped for a variety of reasons, both technical and political," Hashash says. The advent of modern boring machines and the indisputable economic benefit of joining the two countries helped bring the concept to fruition, resulting in one of the most impressive demonstrations of engineering prowess of all time. To account for hazards such as fires—and there have been several within the tunnel since it was built—the designers built a smaller service tunnel in between the two main tubes to act as an escape route.
3. Zion-Mount Carmel TunnelLocation /// Zion Canyon, UtahBackground: Drivers will encounter this scenic structure between Utah's Mount Zion National Park and Bryce Canyon National Park. Construction dates back to the formative years of the U.S. National Park Service, and a dedication ceremony opened the tunnel to traffic on July 4, 1930.Why It's Unique: A series of windows carved though the tunnel's side open up breathtaking views for drivers. "As you leave Zion heading to go into Bryce, you climb up this road and go through this beautiful tunnel. You can see the rock formations and the scenery is just gorgeous," Hashash says. Because the tunnel is built through sandstone, a relatively soft rock, engineers have had to reinforce the structure several times during the past 80 years, adding concrete ribs and an around-the-clock monitoring system.
4. Smuggling TunnelLocation /// Tijuana, MexicoBackground: Crafty criminals have used underground tunnels to commit all sorts of crimes, including bank robberies and drug smuggling. During the past decade, a slew of so-called smuggling tunnels have been discovered on U.S. soil, coming from both Canada and Mexico.Why It's Unique: In December, U.S. and Mexican officials uncovered this 1000-foot-long tunnel, which ran 860 feet into the U.S. The tunnel, though not completed, was outfitted with lighting, a ventilation system, electricity, and even an elevator. A statement from the Drug Enforcement Agency indicates that this DIY tunnel was under construction for two years.
5. Marmaray TunnelLocation /// Istanbul, TurkeyBackground: Construction of this intercontinental tunnel started in 2004, but archaeological discoveries from the Byzantine era and other hurdles have delayed its completion.Why It's Unique: This immersed tube tunnel will be the first rail tunnel joining two different continents, running under the Bosporus Strait to link the European side of Istanbul with the Asian side. Hashash explains that the immersed-tube design requires large portions of the tunnel to be built on a dry dock that is floated out into the water and sunk. Marmaray Tunnel's maximum depth of 180 feet will make it the deepest immersed tube tunnel in the world. 
6. Thames TunnelLocation /// London, EnglandBackground: At first glance the Thames Tunnel doesn't seem particularly impressive; its 1300-foot length and 75-foot depth pale in comparison to many other tunnels. But the Thames Tunnel is a historical milestone in tunnel engineering.Why It's Unique: "This was the first tunnel going under a body of water, and it was the first time the shield tunneling concept was used," Hashash tells PM. "It is a major milestone." The tunneling shield was a device developed by Sir Marc Isambard Brunel that acts as a temporary support structure, allowing construction crews to safely, and dryly, install permanent support systems underwater. Building the Thames Tunnel was such a novelty in the early 19th century that visitors would pay to walk inside the construction site, and Brunel even hosted concerts and banquets inside the unfinished structure. When the tunnel was completed in 1843, Queen Victoria knighted Brunel for his world-changing contribution to engineering.
7. Seikan TunnelLocation /// Honshu, JapanBackground: This 33.5-mile-long rail tunnel links the island of Honshu with the island of Hokkaido. Construction was completed in 1988.Why It's Unique: At 787 feet below sea level, the Seikan Tunnel is the deepest tunnel in the world. Additionally, its 33.5-mile length makes it the longest tunnel currently in use. The region's unpredictable geology and its volcanic rock prevented engineers from using boring machines, forcing them to blast and dig their way under the Tsugaru Strait.
8. Yerba Buena Island TunnelLocation /// Bay Area, CaliforniaBackground: Yerba Buena Island is a small chunk of land resting in the San Francisco Bay, between Oakland and San Francisco. A cantilevered bridge flanks this five-lane tunnel on one side, with a double suspension bridge on the other.Why It's Unique: With a 76-foot diameter, Yerba Buena Island Tunnel is the world's widest single-bore tunnel, an impressive feat considering it opened in November of 1936. Hashash speculates that adding the tunnel was the most cost-effective solution to allowing a passage through the island, as opposed to just building one enormous bridge.
9. Infiltration TunnelsLocation /// 38th Parallel DMZBackground: Three different tunnels were discovered creeping under Korea's Demilitarized Zone, one of the most contentious borders in the world, throughout the 1970s. All of them appeared to originate in North Korea, and later, in 1990, a fourth tunnel was discovered.Why It's Unique: The Third Invasion Tunnel, also referred to as the Third Tunnel of Aggression, came within 26 miles of Seoul, South Korea's capital city. Clandestine, underground routes provide North Korea the means to launch a massive military offensive, and the tunnels were large enough to shuttle through an entire military division per hour. GlobalSecurity.org estimates that as many as 20 tunnels encroach on the DMZ; a frightening number given the precarious relationship between North and South Korea.
10. Natural TunnelLocation /// Natural Tunnel State Park, VirginiaBackground: Construction of this tunnel dates back millions of years, when carbonic acid began eating away at the limestone and dolomite bedrock. An article published in a geology journal in 1832 is the first documentation of the tunnel's existence.Why It's Unique: The tunnel's 200-foot width is large enough to accommodate trains, so in 1906 Southern Railway established a passenger line that snaked under the natural structure. Today, the passenger rail is no longer in use, but freights continue to haul coal through the tunnel on a regular basis. The awe-inducing nature of the structure led William Jennings Bryan, 1896 presidential candidate and Woodrow Wilson's secretary of state, to declare it the eighth wonder of the world.
11. Eisenhower–Johnson Memorial TunnelLocation /// Clear Creek County, ColoradoBackground: Drivers will find this 1.67-mile-long tunnel about an hour west of Denver in the midst of the Rockies. The westbound bore opened in 1973, saving commuters close to an hour by providing them an alternate route through the formidable Loveland Pass.Why It's Unique: There's a lot that makes the Eisenhower Tunnel stand out, such as its 11,158-foot elevation, which makes it one of the highest tunnels in the world, as well as the fact that it's the longest mountain tunnel in the U.S. interstate system. Adding to the structure's mystique is that it cuts under the Continental Divide, the point at which bodies of water that drain into the Pacific are divided from the bodies of water that drain into the Atlantic.
12. Cheyenne Mountain ComplexLocation /// El Paso County, ColoradoBackground: Deep in the bowels of Cheyenne Mountain is a city-like military complex that once served as North American Aerospace Defense Command (NORAD)'s center. Designed during the height of the Cold War, the facility is intended to withstand nuclear blasts and is equipped with self-sufficient features, like a water supply provided from a spring within the mountain.Why It's Unique: Employees clocking in at the complex take a daily commute down a roughly 2000-foot tunnel, traveling through a few 25-ton blast doors along the way. Building the tunnel required engineers to bore through massive amounts of solid granite. The facility opened in 1966. "Granite is a particularly hard rock," Hashash says. "For a protective structure it's excellent."
13. MailRail
Location /// London, England
Background: Several post offices have experimented with underground mail delivery. The USPS used an elaborate system of pneumatic tubes during the early 19th century, but London took an entirely different approach and built this scaled-down railroad system to be used only for parcels.
Why It's Unique: About 23 miles worth of 2-foot-gauge track run along the MailRail, which is 70 feet below the hustle and bustle of London's streets. During the system's heyday, unmanned electric trains lugged daily loads of 4 million letters. The trains could make it through the 6.5-mile-long tunnel in less than half an hour, stopping at nine different sorting stations along the way. Use of the system was gradually scaled back until 2003, when the tunnels and rails were abandoned because of cost constraints.
14. Tokyo Bay Aqua-LineLocation /// Kawasaki City, JapanBackground: A nearly 3-mile-long bridge dips into this 6-mile-long tunnel in a region prone to earthquakes. The structure opened to traffic in 1997, saving commuters traveling between Kawasaki City and Kisarazu City roughly 45 minutes each way.Why It's Unique: While bridge and tunnel combinations exist throughout the world, a large rest area constructed atop an artificial island at the entrance of this tunnel makes the Aqua-Line stand out among the them. The island, called Umi-Hotaru, which translates into "firefly of the sea," acts primarily as a rest area consisting of shops and restaurants, and an observation deck that looks out on the bay.
15. PATH Tunnel SystemLocation /// Toronto, CanadaBackground: Canadian winters can be brutal, but this enormous underground complex provides commuters with a quick and warm way to get around downtown Toronto. Over 100,000 daily commuters pass through the mall-like system, which is home to more than 1200 retail shops, every day.Why It's Unique: Several cities, including Houston, Minneapolis and Montreal, have similar underground systems, but Toronto's 16 miles worth of tunnels makes it the largest. Accounting for psychological factors is an important consideration for these complexes, Hashash says, especially when it comes to orienting pedestrians with their exact whereabouts. Toronto developed a comprehensive signage system for its tunnel complex during the 1990s, and each letter in PATH is color-coded and represents a different direction. Commuters walking through the red P-section can be assured they are heading south, while an orange A would indicate that they're traveling west.
16. Cu Chi TunnelsLocation /// Cu Chi, VietnamBackground: This expansive labyrinth of tunnels played an important role in the Vietcong's strategy during both the French Indochina War and the Vietnam War. Estimates on the actual size of the tunnels range from 75 miles to over 150 miles.Why It's Unique: The Viet Cong constructed tunnel systems throughout Vietnam, but the tunnels in Cu Chi have become a popular tourist destination in recent years. Visitors can work their way through the cramped space, check out deadly booby traps and even visit an underground command center where the Tet offensive was planned. Some of the tunnels are just a few feet tall, while others have been retrofitted to accommodate larger tourists.
17. Gotthard Base TunnelLocation /// Erstfeld, SwitzerlandBackground: This work in progress will be the world's longest transportation tunnel—coming in at over 35 miles long—when it is completed. Drilling through the Swiss Alps is no easy task, so the timeline for completion has been pushed back several years, and the price has increased by billions of euros.Why It's Unique: Despite the mountainous topography of the region, trains traveling through the Gotthard Base Tunnel will remain at nearly the same elevation for the entire trip. "The need to maintain elevation is an important consideration if you want to run higher-speed rails through the tunnel," Hashash notes. And while the cost of the project has soared, the tunnel could play a vital role in boosting trade between Switzerland, Italy, Germany, Austria and many other European countries.
18. Jiucyudong TunnelLocation /// Taroko National Park, TaiwanBackground: This twisting, man-made structure is also known as the Tunnel of Nine Turns, a reference to the coiled dragons popular in Chinese folklore and mythology.Why It's Unique: The Tunnel of Nine Turns dices through marble cliffs and runs along a river bank in Taroko National Park, an area known for its rich geology, including a form of jade endemic to Taiwan. Completed in 1996, the structure gives tourists a unique glimpse of natural marble formations, though falling rocks do pose a real threat—and helmets may be required for sightseers.
