An underwater tunnel is a

tunnel A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube cons ...

which is partly or wholly constructed under the sea or a river. They are often used where building a bridge or operating a ferry link is unviable, or to provide competition or relief for existing bridges or ferry links.Sullivan, Walter
Progress In Technology Revives Interest In Great Tunnels

New York Times ''The New York Times'' (''the Times'', ''NYT'', or the Gray Lady) is a daily newspaper based in New York City with a worldwide readership reported in 2020 to comprise a declining 840,000 paid print subscribers, and a growing 6 million paid d ...

, June 24, 1986. Retrieved 15 August 2010. While short tunnels are often road tunnels which may admit motorized traffic, unmotorized traffic or both, concerns with ventilation lead to the longest tunnels (such as the

Channel Tunnel The Channel Tunnel (french: Tunnel sous la Manche), also known as the Chunnel, is a railway tunnel that connects Folkestone (Kent, England, UK) with Coquelles ( Hauts-de-France, France) beneath the English Channel at the Strait of Dover. ...

or the

Seikan Tunnel The Seikan Tunnel ( ja, 青函トンネル, or , ), is a dual-gauge railway tunnel in Japan, with a portion under the seabed of the Tsugaru Strait, which separates Aomori Prefecture on the main Japanese island of Honshu from the northern isla ...

) being

electrified Electrification is the process of powering by electricity and, in many contexts, the introduction of such power by changing over from an earlier power source. The broad meaning of the term, such as in the history of technology, economic history ...

rail tunnel A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube constr ...

Types of tunnel

Various methods are used to construct underwater tunnels, including an

immersed tube An immersed tube (or immersed tunnel) is a kind of undersea tunnel composed of segments, constructed elsewhere and floated to the tunnel site to be sunk into place and then linked together. They are commonly used for road and rail crossings of r ...

and a

submerged floating tunnel A submerged floating tunnel (SFT), also known as submerged floating tube bridge (SFTB), suspended tunnel, or Archimedes bridge, is a proposed design for a tunnel that floats in water, supported by its buoyancy (specifically, by employing the hydro ...

. The immersed tube method involves steel tube segments that are positioned in a trench in the sea floor and joined together. The trench is then covered and the water pumped from the tunnel. Submerged floating tunnels use the law of buoyancy to remain submerged, with the tunnel attached to the sea bed by columns or tethers, or hung from pontoons on the surface.

Advantages

Compared with bridges

One such advantage would be that a tunnel would still allow shipping to pass. A low bridge would need an opening or swing bridge to allow shipping to pass, which can cause

traffic congestion Traffic congestion is a condition in transport that is characterized by slower speeds, longer trip times, and increased vehicular queueing. Traffic congestion on urban road networks has increased substantially since the 1950s. When traffic de ...

. Conversely, a higher bridge that does allow shipping may be unsightly and opposed by the public. Higher bridges can also be more expensive than lower ones. Bridges can also be closed due to harsh weather such as high winds. Tunneling makes excavated soil available that can be used to create new land (see

land reclamation Land reclamation, usually known as reclamation, and also known as land fill (not to be confused with a waste landfill), is the process of creating new land from oceans, seas, riverbeds or lake beds. The land reclaimed is known as reclamati ...

). This was done with the rock excavated for the

, which was used to create

Samphire Hoe Samphire Hoe is a country park situated west of Dover in Kent in southeast England. The park was created by using 4.9 million cubic metres of chalk marl from the Channel Tunnel excavations and is found at the bottom of a section of the White Cl ...

Compared with ferry links

As with bridges, albeit with more chance, ferry links will also be closed during adverse weather. Strong winds or the tidal limits may also affect the workings of a ferry crossing. Travelling through a tunnel is significantly quicker than travelling using a ferry link, shown by the times for travelling through the

(75–90 minutes for Ferry and 21 minutes on the

Eurostar Eurostar is an international high-speed rail service connecting the United Kingdom with France, Belgium and the Netherlands. Most Eurostar trains travel through the Channel Tunnel between the United Kingdom and France, owned and operated sep ...

). Ferries offer much lower frequency and capacity and travel times tend to be longer with a ferry than a tunnel. Ferries also usually use

fossil fuels A fossil fuel is a hydrocarbon-containing material formed naturally in the Earth's crust from the remains of dead plants and animals that is extracted and burned as a fuel. The main fossil fuels are coal, oil, and natural gas. Fossil fuels ...

emitting

greenhouse gases A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), methane ...

in the process while most railway tunnels are

. In the

Baltic Sea The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden and the North and Central European Plain. The sea stretches from 53°N to 66°N latitude and from ...

, one of the busiest areas for

passenger ferries A ferry is a ship, watercraft or amphibious vehicle used to carry passengers, and sometimes vehicles and cargo, across a body of water. A passenger ferry with many stops, such as in Venice, Italy, is sometimes called a water bus or water taxi ...

in the world,

sea ice Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oce ...

is a problem, causing seasonal disruption or requiring expensive ice-breaking ships. In the

Øresund region The Øresund Region ( da, Øresundsregionen ; sv, Öresundsregionen ), also known as Greater Copenhagen for marketing purposes, is a metropolitan region that comprises eastern Denmark and Skåne in southern Sweden. Centred around the Øresund ...

the construction of the bridge-tunnel has been cited as enhancing regional integration and giving an economic boom not possible with the previous ferry links. Similar arguments are used by proponents of the Helsinki-Tallinn tunnel in the

Talsinki Talsinki (sometimes also Hellinna or Hellinn) is a colloquial name for geographical region of Helsinki, Finland and Tallinn, Estonia Estonia, formally the Republic of Estonia, is a country by the Baltic Sea in Northern Europe. It is bor ...

region. There are various issues with the safety of both tunnels and ferries, in the case of tunnels, fire is a particular hazard with several fires having broken out in the Channel Tunnel. On the other hand, the

free surface effect The free surface effect is a mechanism which can cause a watercraft to become unstable and capsize. It refers to the tendency of liquids — and of unbound aggregates of small solid objects, like seeds, gravel, or crushed ore, whose behavior ap ...

is a significant safety risk for

RORO Roll-on/roll-off (RORO or ro-ro) ships are cargo ships designed to carry wheeled cargo, such as cars, motorcycles, trucks, semi-trailer trucks, buses, Trailer (vehicle), trailers, and railroad cars, that are driven on and off the ship on their o ...

ferries as seen in the sinking of

MS Estonia MS ''Estonia'' was a cruiseferry built in 1980 at the West German shipyard Meyer Werft in Papenburg. In 1993, she was sold to Nordström & Thulin for use on Estline's Tallinn–Stockholm route. The ship's sinking on 28 September 1994, in the B ...

. Tunnels which exclude dangerous, combustible freights and the fuel carried aboard motorcars can significantly reduce fire risk.

Disadvantages

Compared with bridges

Tunnels require far higher costs of security and construction than bridges. This may mean that over short distances bridges may be preferred rather than tunnels (for example

Dartford Crossing The Dartford-Thurrock River Crossing, commonly known as the Dartford Crossing and until 1991 the Dartford Tunnel, is a major road crossing of the River Thames in England, carrying the A282 road between Dartford in Kent in the south and Thurr ...

). As stated earlier, bridges may not allow shipping to pass, so solutions such as the

Øresund Bridge The Öresund or Øresund Bridge ( da, Øresundsbroen ; sv, Öresundsbron ; hybrid name: ) is a combined railway and motorway bridge across the Øresund strait between Denmark and Sweden. It is the longest in Europe with both roadway and rai ...

have been constructed.

Compared with ferry links

As with bridges, ferry links are far cheaper to construct than tunnels, but not to operate. Also tunnels don't have the flexibility to be deployed over different routes as transport demand changes over time. Without the cost of a new ferry, the route over which a ferry provides transport can easily be changed. However, this flexibility can be a downside for customers who have come to rely on the ferry service only to see it abandoned. Fixed infrastructure such as bridges or tunnels represent a much more concrete commitment to sustained service.

List of notable examples

Proposed

Road

Rogfast tunnel Rogaland Fixed Link or simply the Rogfast is a project, constructing a sub-sea road tunnel between the municipalities of Randaberg (near the Stavanger (city), city of Stavanger) and Bokn in Rogaland county, Norway. The tunnel will be called the B ...

in Norway – construction having started in 2018, at 27 km length, 392 m depth, it will be the longest road tunnel and deepest undersea tunnel in the world. * Underwater Road Tunnel Salamina island-Perama - planned road tunnel in

Attica Attica ( el, Αττική, Ancient Greek ''Attikḗ'' or , or ), or the Attic Peninsula, is a historical region that encompasses the city of Athens, the capital of Greece and its countryside. It is a peninsula projecting into the Aegean Se ...

Greece Greece,, or , romanized: ', officially the Hellenic Republic, is a country in Southeast Europe. It is situated on the southern tip of the Balkans, and is located at the crossroads of Europe, Asia, and Africa. Greece shares land borders with ...

. Currently at the second stage of the tender from which the concessionaire will be selected. * India-Sri Lanka Sea Tunnel ''(proposed)'' *

Penang Undersea Tunnel The Penang Undersea Tunnel is a tunnel under construction between Butterworth, Penang, Butterworth on the mainland and George Town, Penang, George Town on the island of Penang in northern Malaysia. Construction began in 2016. The 7.2 km tun ...

Malaysia Malaysia ( ; ) is a country in Southeast Asia. The federation, federal constitutional monarchy consists of States and federal territories of Malaysia, thirteen states and three federal territories, separated by the South China Sea into two r ...

– to be opened in 2025

Rail

Bohai Strait tunnel The Bohai Strait Tunnel or Dalian-Yantai Tunnel is a proposed undersea tunnel construction project across the Bohai Strait to connect Dalian on the Liaodong Peninsula with Yantai on the Shandong Peninsula. The official name for the project is ...

in China between

Dalian Dalian () is a major sub-provincial port city in Liaoning province, People's Republic of China, and is Liaoning's second largest city (after the provincial capital Shenyang) and the third-most populous city of Northeast China. Located on the ...

and

Yantai Yantai, Postal Map Romanization, formerly romanization of Chinese, known as Chefoo, is a coastal prefecture-level city on the Shandong Peninsula in northeastern Shandong province of People's Republic of China. Lying on the southern coast of ...

(decided, construction to start 'as soon as possible'.) * Helsinki to Tallinn Tunnel under the

Gulf of Finland The Gulf of Finland ( fi, Suomenlahti; et, Soome laht; rus, Фи́нский зали́в, r=Finskiy zaliv, p=ˈfʲinskʲɪj zɐˈlʲif; sv, Finska viken) is the easternmost arm of the Baltic Sea. It extends between Finland to the north and E ...

''(proposed)'' *

Irish Sea Tunnel There are a number of proposed fixed connections—road or rail, bridge or tunnel—designed to connect the islands of Ireland and Great Britain, connect the island of Great Britain to mainland Europe, as well as to build other connections amongs ...

''(suggested)'' *

Rio de Janeiro Metro The Rio de Janeiro Metro ( pt, MetrôRio, ), commonly referred to as just the ''Metrô'' () is a rapid transit network that serves the city of Rio de Janeiro, Brazil. The Metrô was inaugurated on 5 March 1979, and consisted of five stations op ...

Bay Tunnel (Line 3 –

Rio de Janeiro Rio de Janeiro ( , , ; literally 'River of January'), or simply Rio, is the capital of the state of the same name, Brazil's third-most populous state, and the second-most populous city in Brazil, after São Paulo. Listed by the GaWC as a b ...

Niterói Niterói (, ) is a Municipalities of Brazil, municipality of the state of Rio de Janeiro (state), Rio de Janeiro in the Southeast Region, Brazil, southeast region of Brazil. It lies across Guanabara Bay facing the city of Rio de Janeiro and forms ...

) (proposed) *

Fehmarn Belt Fixed Link The Fehmarn Belt fixed link ( da, Femern Bælt-forbindelsen, german: Fehmarnbelt-Querung) or Fehmarn Belt tunnel is an under-construction immersed tunnel, which will connect the Danish island of Lolland with the German island of Fehmarn, cros ...

between Denmark and Germany (decided, construction started in January 2021) *

Mumbai–Ahmedabad high-speed rail corridor Mumbai–Ahmedabad High Speed Rail Corridor (MAHSR) or Mumbai–Ahmedabad HSR is an under-construction high-speed rail line connecting India's economic hub Mumbai with the city of Ahmedabad. When completed, it will be India's first high-speed r ...

of India (decided, construction start November 2018) * India-Sri Lanka Sea Tunnel ''(proposed)'' * Taiwan Strait Tunnel - if built would become the longest rail tunnel in the world engineering challenges and the unsolved

political status of Taiwan The controversy surrounding the political status of Taiwan or the Taiwan issue is a result of World War II, the second phase of the Chinese Civil War (1945–1949), and the Cold War. The basic issue hinges on who the islands of Taiwan, Peng ...

make construction unlikely *

Strait of Gibraltar Tunnel The Strait of Gibraltar crossing is a hypothetical bridge or tunnel spanning the Strait of Gibraltar (about 14 km or 9 miles at its narrowest point) that would connect Europe and Africa. The governments of Spain and Morocco appointed a joi ...

- linking Gibraltar or the Spanish mainland to the African mainland. If built it would most likely become the deepest tunnel ever built

References

{{Authority control * Coastal construction

Types of tunnel

Advantages

Compared with bridges

Compared with ferry links

Disadvantages

Compared with bridges

Compared with ferry links

List of notable examples

Proposed

Road

Rail

See also

References