Constructed partially or entirely beneath bodies of water, underwater tunnels serve as vital infrastructure solutions where constructing bridges or establishing ferry routes is impractical. They offer competition or alleviation for existing transportation links. Short tunnels typically accommodate road traffic, including both motorised and non-motorised vehicles. In contrast, ventilation considerations dictate that the longest tunnels, such as the Channel Tunnel or the Seikan Tunnel, are predominantly utilised for electrified rail traffic.
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Underwater Tunnels in the World for Train, Metro (2024)
Diverse techniques are employed in the construction of underwater tunnels, encompassing both the immersed tube and submerged floating tunnel methods. The immersed tube approach entails the placement of steel tube segments within a trench on the seabed, subsequently interconnected. Following this, the trench is enclosed, and water is drained from the tunnel. Conversely, submerged floating tunnels leverage buoyancy principles to remain submerged. These tunnels are affixed to the seabed through columns or tethers or suspended from pontoons on the water's surface:
| 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 | Constructed in 1931, the CESC Tunnel holds the distinction of being both Asia's and India's maiden underwater tunnel. Serving as a conduit for electric power transmission between Kolkata and Howrah, it represents a pioneering feat in the region's infrastructure landscape. | 0.539 km | 33.5 m | 1931 |
| Tongyeong Undersea Tunnel | Tongyeong, South Korea | Tongyeong Undersea Tunnel, inaugurated in 1932, stands as Asia's premier undersea tunnel, linking Tongyeong with Mireukdo Island. | 0.483 km | 13.5 m | 1932 |
| Queensway Tunnel | Liverpool, England | When it was initially constructed, the Queensway Tunnel held the distinction of being the world's longest vehicular tunnel of any kind. Spanning the Mersey estuary, it provided a vital connection between Liverpool and Birkenhead, setting a new standard for vehicular tunnel infrastructure worldwide. | 3.24 km | 1925–1934 | |
| Bankhead Tunnel | Mobile, Alabama | The Bankhead Tunnel, serving Highway 90 in Mobile, Alabama, connects the Business District to Blakely Island. Notably, its eastern end features a sizable "flood door" designed to safeguard against water intrusion from Mobile Bay during hurricanes or tropical storms. Currently, the tunnel restricts passage to cars and pickup trucks, with two dedicated lanes for these vehicles. | 1.033 km | 12.2 m | 1938–1942 |
| Kanmon Railway Tunnel | Kanmon Straits, Japan | The Kanmon Railway Tunnel, the first undersea tunnel in Japan, links the islands of Honshu and Kyushu. Situated beneath the Kanmon Straits, it represents one of three tunnels traversing this strategic waterway, symbolising a significant advancement in the country's transportation infrastructure. | 3.604 km | 1936–1942 | |
| Lincoln Tunnel | New York, USA | The Lincoln Tunnel, a series of road tunnels constructed in three phases, spans the Hudson River, connecting Manhattan and New Jersey. This vital transportation link represents a feat of engineering, facilitating seamless travel between the two bustling urban centres. | 2.4 km average | 30 m | 1934–1957 |
| Kanmon Roadway Tunnel | Kanmon Straits, Japan | The Kanmon Roadway Tunnel, the second of three tunnels beneath the Kanmon Straits, serves as a crucial link between Honshu and Kyushu islands in Japan. Upon completion, it held the distinction of being the world's longest undersea road tunnel. Notably, it accommodates pedestrians and cyclists with a dedicated tunnel pathway. | 3.461 km | 58 m | 1937–1958 |
| Havana Tunnel | Havana, Cuba | The Havana Tunnel, constructed during the Batista era, traverses Havana Bay. | 0.733 km | 12 m | 1957–1958 |
| George Massey Tunnel | Vancouver, Canada | The George Massey Tunnel is the first tunnel in British Columbia to utilise Immersed Tube technology. This innovative construction method enhances the tunnel's structural integrity and efficiency, marking a significant advancement in the province's transportation infrastructure. | 0.629 km | 23 m | 1957–1959 |
| Muskö Tunnel | Muskö, Sweden | The Muskö Tunnel links Muskö Island to the mainland in the Stockholm coastal region. This crucial passage enhances accessibility and connectivity for residents and travelers, facilitating smoother transportation between the island and the mainland, thus contributing to regional development and economic growth. | 2.9 km | 65 m | 1959–1964 |
| Sandoyartunnilin | Sandoy, Faroe Islands | Connecting Sandoy to the main island of Eysturoy. | 10.8 km | 155 m | 2018–2023 |
| Chesapeake Bay Bridge-Tunnel | Virginia, USA | Connects Virginia Beach with the Eastern Shore of Virginia. | 1.6 km | - | 1960–1964 |
| Transbay Tube | San Francisco – Oakland, USA | Rail tunnel for Bay Area Rapid Transit. Connects Oakland to San Francisco. | 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 | The Ahmed Hamdi Tunnel traverses beneath the Suez Canal, establishing a vital link between the Asian Sinai Peninsula and the town of Suez on the African mainland. Notably, it comprises seven tunnels, including two for railways, four for roads, and one serving other purposes, facilitating seamless transportation and trade between the two continents. | 1.63 km | - | 1979–1981 |
| Vardø Tunnel | Vardo, 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 |
| Channel Tunnel | England – France | The world's longest undersea portion railway tunnel (37.9 km underwater length) | 50.4 km | - | 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 Massachusetts Bay Outfall serves as the discharge point for the Deer Island Treatment Plant, redirecting treated sewage into Massachusetts Bay instead of the shallower waters of Boston Harbour. With a diameter of 24 feet 3 inches (7.39 metres), this tunnel plays a critical role in environmental conservation by effectively managing wastewater disposal in the region. | 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 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 | The road tunnel serves as a link between Reykjavik and the 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 |
Excavating tunnels yields surplus soil that can be repurposed for land reclamation endeavours. An exemplary instance of this practice occurred during the construction of the Channel Tunnel, where the excavated rock was utilised to establish Samphire Hoe.
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PM Modi Inaugurates India's First Underwater Metro
Prime Minister Narendra Modi unveiled India's inaugural underwater metro route in Kolkata, underscoring a momentous stride in the nation's infrastructure advancement. This submerged service constitutes a segment of the Howrah Maidan-Esplanade section within the Kolkata Metro's East-West corridor, stretching 16.6 kilometres beneath the Hooghly River.
Linking Howrah and Salt Lake, the neighbouring municipalities of West Bengal's state capital, this metro route boasts three subterranean stations. Remarkably, it is projected to traverse a 520-metre expanse beneath the Hooghly River in a swift 45 seconds.
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