Gotthard Base Tunnel (Switzerland)
Opened in 2016, this roughly 57 km railway tunnel is considered the longest and deepest transport tunnel in the world. It passes through the Alps to connect northern and southern Switzerland, allowing freight and passengers to be transported far faster and more safely than the previous rail routes that wound over the mountains.
Seikan Tunnel (Japan)
An undersea rail tunnel connecting Honshu and Hokkaido, Japan; it is known as the longest undersea tunnel in the world in terms of the length of its undersea section alone. It was famously extremely difficult to construct due to having to pass beneath a rough strait, and since its completion it has served as a key land route safely linking the two islands.
Channel Tunnel
An undersea tunnel connecting the United Kingdom and France beneath the Strait of Dover, opened in 1994. The high-speed Eurostar train runs through this tunnel, allowing travel between London and Paris overland without a ferry or a flight, and it is often cited as a symbolic piece of infrastructure for European integration.
Lærdal Tunnel (Norway)
At roughly 24.5 km, this is the longest road tunnel in the world, located in Norway, a country with many fjords. Norway is known for having an unusually large number of long road tunnels because of its rugged fjord terrain, and it is also known for building wide, differently lit chambers at intervals inside its tunnels to help reduce driver fatigue and tension.
Delaware Aqueduct (United States)
A water-supply tunnel that delivers water to New York City. Although it is not used for transport, in terms of length alone it is considered one of the longest tunnels in the world. It is not a tunnel that people or vehicles pass through, but it is a classic example of a tunnel whose scale and importance — as core infrastructure supplying an entire city with water — is not widely known.
How tunnels are built
Mountains and solid bedrock are often bored through using massive tunnel boring machines (TBMs), while undersea or underwater sections are sometimes built using the immersed tube method, in which prefabricated structures are sunk and connected. Construction methods vary greatly depending on terrain and geological conditions, and even a single tunnel may combine different methods across different sections.
Tunnels in Korea
Korea, with its mountainous terrain, is known for having a high density of road and rail tunnels, used in a variety of forms from urban subway tunnels to mountain tunnels for high-speed rail. Because most of the country's land is mountainous, tunnel construction has long played a key role in expanding Korea's road and rail networks.