# Underwater tunnel

An underwater tunnel is a tunnel 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. 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 or the Seikan Tunnel) being electrified rail tunnels.

## Types of tunnel

Various methods are used to construct underwater tunnels, including an immersed tube and a submerged floating tunnel. 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. 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). This was done with the rock excavated for the Channel Tunnel, which was used to create Samphire Hoe.

### 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 Channel Tunnel (75–90 minutes for Ferry and 21 minutes on the Eurostar). 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 emitting greenhouse gases in the process while most railway tunnels are electrified. In the Baltic Sea, one of the busiest areas for passenger ferries in the world, sea ice is a problem, causing seasonal disruption or requiring expensive ice-breaking ships. In the Øresund region 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 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 is a significant safety risk for RORO ferries as seen in the sinking of MS Estonia. Tunnels which exclude dangerous, combustible freights and the fuel or lithium-ion batteries 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). As stated earlier, bridges may not allow shipping to pass, so solutions such as the Øresund Bridge 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

| Name | Place | Description | Length | Depth (from surface) | Constructed in |
| Thames Tunnel | London, England | Thought to be the oldest tunnel under a navigable river, crossing the Thames in London | 0.4 km | | 1825–1843 |
| Mersey Railway Tunnel | Liverpool, England | The oldest underwater rail tunnel in the world, crossing the Mersey in Liverpool | 1.21 km | | 1881–1886 |
| Severn Tunnel | Wales – England | One of the oldest underwater rail tunnels in the world | 7.01 km | | 1873–1886 |
| Blackwall Tunnel (western) | London, England | The oldest underwater vehicular tunnel in the world, crossing the Thames in London | 1.35 km | | 1892–1897 |
| Elbe Tunnel (1911) | Hamburg, Germany | Pioneering underwater pedestrian and vehicular tunnel, crossing the Elbe River in Hamburg | 0.426 km | 24 m | 1907–1911 |
| Holland Tunnel | New York – New Jersey, USA | The longest continuous underwater vehicular tunnel in the world when first built, crossing the Hudson River between Manhattan and Jersey City | 2.6 km | 28.3 m | 1920–1927 |
| Detroit–Windsor Tunnel | Windsor, Canada – Detroit, USA | Connect Ontario, Canada to Michigan, USA. under the Detroit River opened on November 3, 1930 | 1.57 km | 13.7 m | 1928–1930 |
| CESC Tunnel | Kolkata, India | It is the first underwater tunnel of Asia, as well as India. The construction of this tunnel was completed in 1931. This tunnel is used for electric power transmission between Kolkata and Howrah. | 0.539 km | 33.5 m | 1931 |
| Tongyeong Undersea Tunnel | Tongyeong, South Korea | The first undersea tunnel in Asia, connecting Tongyeong to the Mireukdo island | 0.483 km | 13.5 m | 1932 |
| Queensway Tunnel | Liverpool, England | The longest vehicular tunnel of any type in the world when first built, crossing the Mersey estuary between Liverpool and Birkenhead | 3.24 km | | 1925–1934 |
| Bankhead Tunnel | Mobile, Alabama | Carries Hwy. 90 in Mobile, AL. Business District, to Blakely Island. The eastern end has large "flood door" that can be closed to prevent water from the Mobile Bay from flooding the tunnel during hurricanes or tropical storms. Two lanes that only allows cars and pick up trucks now to travel through the tunnel | 1.033 km | 12.2 m | 1938–1942 |
| Kanmon Railway Tunnel | Kanmon Straits, Japan | The first undersea tunnel in Japan, connecting the islands of Honshu and Kyushu. One of three tunnels underneath the Kanmon Straits | 3.604 km | | 1936–1942 |
| Lincoln Tunnel | New York, USA | Set of road tunnels built in three stages, crossing the Hudson River between Manhattan and New Jersey | 2.4 km average | 30 m | 1934–1957 |
| Kanmon Roadway Tunnel | Kanmon Straits, Japan | The second of three tunnels under the Kanmon Straits, connecting the islands of Honshu and Kyushu. It was the world's longest undersea road tunnel at the time of its construction and includes a tunnel for pedestrians and cyclists | 3.461 km | 58 m | 1937–1958 |
| Havana Tunnel | Havana, Cuba | Road tunnel built during the Batista era, crossing the Havana Bay | 0.733 km | 12 m | 1957–1958 |
| George Massey Tunnel | Vancouver, Canada | The first tunnel in British Columbia to use Immersed Tube technology | 0.629 km | 23 m | 1957–1959 |
| Muskö Tunnel | Muskö, Sweden | Connecting Muskö island to mainland in Stockholm coastal region | 2.9 km | 65 m | 1959–1964 |
| Sandoyartunnilin | Sandoy, Faroe Islands | Connecting Sandoy to the main island Eysturoy | 10.8 km | 155 m | 2018–2023 |
| Chesapeake Bay Bridge Tunnel | Virginia, USA | Connects Virginia Beach with the Eastern Shore of Virginia. Length refers to the tunnel section | 1.6 km | | 1960–1964 |
| Transbay Tube | San Francisco – Oakland, USA | Rail tunnel for Bay Area Rapid Transit. Connects Oakland to San Francisco. It is the longest underwater tunnel in North America | 5.8 km | 41 m | 1965–1969 |
| Cross-Harbour Tunnel | Hong Kong | A busy road tunnel in Hong Kong | 1.86 km | | 1969–1972 |
| Elbe Tunnel (1975) | Hamburg, Germany | 8-lane road tunnel crossing the Elbe River in Hamburg | 3.3 km | | 1968–1975 |
| Ahmed Hamdi Tunnel | Suez, Egypt | Passes under Suez Canal connecting the Asian Sinai Peninsula to the town of Suez on the African mainland 7 tunnels (2 rail, 4 road, 1 other) under New Suez Canal | 1.63 km | | 1979–1981 |
| Vardø Tunnel | Vardø, Norway | Connecting the small island community of Vardø in northern Norway to the mainland | 2.9 km | 88 m | 1979–1982 |
| Kanonersky Tunnel | Saint-Petersburg, Russia | Connects Kanonersky Island to the Kirovsky District of Saint-Petersburg through Neva Bay | 0.927 km | | 1975–1983 |
| Seikan Tunnel | Seikan, Japan | The Seikan Tunnel is the world's longest tunnel with an undersea segment | 53.8 km | 240 m | 1971–1988 |
| Flekkerøy Tunnel | Flekkerøy, Norway | Connecting the island community of Flekkerøy in southern Norway to the mainland | 2.3 km | 101 m | 1986–1989 |
| Sydney Harbour Tunnel | Sydney, Australia | | 2.8 km | | 1988–1992 |
| Channel Tunnel | England – France | The world's longest undersea portion railway tunnel (37.9 km underwater length) | 50.4 km | 115 m | 1988–1994 |
| Hitra Tunnel | Trøndelag, Norway | The deepest in the world at the time of construction | 5.6 km | 264 m | 1992–1994 |
| Tokyo Bay Aqua-Line | Tokyo, Japan | The world's 2nd longest undersea portion road tunnel | 9.6 km | | 1988–1997 |
| Massachusetts Bay Outfall | Boston, USA | The outfall for the Deer Island Treatment Plant. It discharges treated sewage into Massachusetts Bay instead of into the shallower waters of Boston Harbor. Tunnel diameter 24 feet 3 inches (7.39 m) | 15.3 km | 120 m | 1992–1998 |
| North Cape Tunnel | Magerøya, Norway | Under Magerøysundet, a strait between the Norwegian mainland and the large island of Magerøya and the North Cape, Norway | 6.8 km | 212 m | 1993–1999 |
| Bømlafjord Tunnel | Føyno – Sveio, Norway | The deepest point of the International E-road network. Connects Stord municipality to the Norwegian mainland | 7.8 km | 260.4 m | 1997–2000 |
| Eiksund Tunnel | Møre og Romsdal, Norway | The world's second deepest undersea road tunnel (before 2019 world's deepest) | 7.7 km | 287 m | 2003–2008 |
| Xiang'an Tunnel | Xiamen, China | | 6.05 km | 70 m | 2005–2010 |
| Busan–Geoje Fixed Link | Busan – Geoje, South Korea | | 3.7 km | 48 m | 2008–2010 |
| Qingdao Jiaozhou Bay Tunnel | Hangdao – Qingdao, China | | 7.808 km | 84.2 m | 2006–2011 |
| Marmaray | Istanbul (Bosphorus strait), Turkey | Rail tunnel connecting Asia and Europe. Length refers to the undersea section | 1.39 km | | 2004–2013 |
| Marina Coastal Expressway | Singapore | Singapore's first undersea tunnel | 5 km | | 2008–2013 |
| Port of Miami Tunnel | Miami, USA | | 2.1 km | | 2010–2014 |
| Eurasia Tunnel | Istanbul (Bosphorus strait), Turkey | Road tunnel connecting Asia and Europe in Istanbul | 5.4 km | 106 m | 2011–2016 |
| Hong Kong–Zhuhai–Macau Bridge | Hong Kong – Macau, China | 55 km-long sea crossing between Hong Kong, Macau and Zhuhai, China. Length refers to the tunnel section | 6.7 km | | 2009–2018 |
| Riachuelo Lot 3 Tunnel | Buenos Aires, Argentina | Outfall tunnel of the Riachuelo System - 2nd world's longest outfall undersea tunnel and 4th world's longest undersea tunnel excavated with TBM | 12 km | 48 m | 2017–2019 |
| The Ryfast Tunnel | Stavanger – Ryfylke, Norway | The longest and deepest undersea tunnel for cars, from Stavanger to Ryfylke | 14.3 km | 293 m | 2013–2020 |
| Eysturoyartunnilin | Faroe Islands | Sea crossing between Hvítanes, Strendur and Saltnes, under the Tangafjørður strait. Includes an underwater roundabout | 11.24 km (overall length) | 187 m | 2017–2020 |
| Tuen Mun–Chek Lap Kok Link | Hong Kong | Sea crossing between Tung Chung and Tuen Mun, Hong Kong. Length refers to the tunnel section. | 5 km | | 2011–2020 |
| Boryeong Undersea Tunnel | Boryeong, South Korea | 5th longest undersea section in the world, connects Boryeong with Wonsan Island | 6.927 km | 80 m | 2012-2021 |
| Musaimeer Outfall Tunnel | Doha, Qatar | Diameter 3.7 m | 10.2 km | 40 m | 2017-2021 |
| Haicang Tunnel | Xiamen, China | | 6.293 km | 73.6 m | 2016–2021 |
| East West Metro Tunnel | Kolkata, India | The biggest and the first underwater river railway tunnel in India. It is a metro railway tunnel connecting Kolkata to Howrah. | 0.520 km | 30 m | 2021 |
| Bangabandhu Sheikh Mujibur Rahman Tunnel | Chittagong, Bangladesh | First under-river road tunnel in South Asia | 3.32 km | 18 - 31 m | 2019–2022 |
| Hvalfjörður Tunnel | Capital Region, Iceland | Road tunnel serving as a link between Reykjavik and western portions of the country. | 5.77 km | 165 m | 1996-1998 |
| Great Belt Fixed Link | Sjælland, Sprogø, Denmark | Rail tunnel serving as a link between Sjælland and Sprogø. | 8 km | 80 m | 1988-1995 |

## Proposed
### Road

* Rogfast tunnel 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.
* Karnaphuli Tunnel or Bangabandhu Sheikh Mujibur Rahman Tunnel in Bangladesh Tunnel is an underwater expressway tunnel in the port city of Chittagong, Bangladesh under the Karnaphuli river.
* Underwater Road Tunnel Salamina island-Perama - planned road tunnel in Attica, Greece. Currently at the second stage of the tender from which the concessionaire will be selected.
* India-Sri Lanka Sea Tunnel (proposed)
* Penang Undersea Tunnel in Malaysia – to open in 2025
* Western Harbour Tunnel in Sydney, New South Wales, Australia – to open in 2028
* Suðuroyartunnilin in the Faroe Islands – at least 25 km in length, it would connect the islands of Suðuroy and Skúgvoy to Sandoy, which is part of the fixed-link interconnected Faroese "mainland".

### Rail

* Bohai Strait tunnel in China between Dalian and Yantai (decided, construction to start 'as soon as possible'.)
* Helsinki to Tallinn Tunnel under the Gulf of Finland (proposed)
* Irish Sea Tunnel (suggested)
* Rio de Janeiro Metro Bay Tunnel (Line 3 – Rio de Janeiro-Niterói) (proposed)
* Fehmarn Belt Fixed Link between Denmark and Germany (decided, construction started in January 2021)
* Mumbai–Ahmedabad high-speed rail corridor of India (decided, construction start November 2018)
* Taiwan Strait Tunnel - if built would become the longest rail tunnel in the world. Engineering challenges and the unsolved political status of Taiwan make construction unlikely
* Strait of Gibraltar Tunnel - linking Gibraltar or the Spanish mainland to the African mainland. If built it would most likely become the deepest tunnel ever built