This year marks the 20th anniversary of the opening of the Channel Tunnel linking France with the United Kingdom, and the 50th anniversary of the deal which led to its creation.
To mark the occasion, we're taking a look at some of the world's greatest tunnels.
Channel Tunnel (UK and France)
Connecting the United Kingdom with continental Europe (it has entrances/exits in Folkestone, Kent, and Pas-de-Calais in northern France), the tunnel has the world's longest undersea section -- 37.9 kilometers (23.5 miles).
Though a marvel of the modern age, it wasn't a new idea when it was built.
French engineer Albert Mathieu proposed a tunnel under the English Channel in 1802, although his plans included an artificial island mid-channel where horse-drawn carriages could make maintenance stops.
"This tunnel defined the term 'mega project,'" says Matt Sykes, tunnel expert and director at engineering company Arup.
"It fundamentally changed the geography of Europe and helped to reinforce high speed rail as a viable alternative to short-haul flights."
Length: 50 kilometers (31 miles)
Fast fact: Though both the English and French put in work to build the Channel Tunnel, the English side tunneled a greater distance.
More info: Euro Tunnel
Laerdal Tunnel (Aurland, Norway)
The Laerdal Tunnel in West Norway is the world's longest road tunnel and cost $153 million to build, which works out at $6,250 per meter.
The length of the tunnel prompted engineers to include various features designed to alleviate claustrophobia and tiredness.
"The sheer length of tunnel -- which takes 20 minutes to drive through -- led to innovation in the use of behavioral science and driver psychology in the design to reduce driver fatigue and improve safety," says Sykes.
"This resulted in large, colorfully lit caverns every six kilometers, providing points of interest and a unique driver experience."
Length: 24.5 kilometers (15.2 miles)
Fast fact: Engineers separated the tunnel into different sections to give the illusion that drivers are traveling through a number of smaller tunnels.
In these smaller sections drivers can take breaks, or even have a wedding ceremony, as one adventurous couple has previously done.
More info: The Fjords
Tokyo Bay Aqua-Line (Tokyo)
It's easy to mistake this tunnel for a bridge because part of the structure comprises a 4.4-kilometer span as well as a 9.6-kilometer subsea conduit.
The Aqua Line crosses Tokyo Bay and connects the cities of Kawasaki and Kisarazu.
It reduced the journey time between the two from 90 to 15 minutes.
"This project required the world's largest undersea tunnel boring machines and set the precedent for constructing two-lane road tunnels," points out Matt Sykes at Arup.
"The resilience of the construction was demonstrated during the 2011 Tohoku-Pacific Ocean earthquake, which caused severe damage to Tokyo Bay."
Length: 14 kilometers total (8.7 miles)
Fast fact: Constructed atop the Tokyo Bay Aqua Line is an island that functions as a rest area and mall.
The man-made island, called Umi-Hotaru, is a popular scenic point with an observation deck that gives a great view of Tokyo Bay.
More info: Nippon Civil Consulting Engineers
Eisenhower Tunnel (Colorado)
Colorado's Eisenhower road tunnel is one of the world's highest, located 3,401 meters (11,158 feet) above sea level, at the highest point on the U.S. interstate highway system.
It played a significant role in the women's rights movement when Janet Bonnema was hired as a construction worker in 1972.
Her supervisor misread her name as James, but realized his mistake and reassigned her to administrative duties after workers -- many of whom were former miners -- cited the common superstition that a woman's presence can bring bad luck to a mine.
Bonnema sued and was allowed to return to the tunnel.
A new equal rights law was subsequently passed.
Length: 2.72 kilometers (1.7 miles)
Fast fact: Prior to the tunnel's official opening in 1972, a drunken driver believed he should be the first person to take a vehicle through and was arrested in connection with trespassing.
Charges were subsequently dropped because the signs prohibiting traffic were considered inadequate.
More info: Colorado Department of Transportation