Question: What is the length of the Seikan Tunnel that is the world 's longest tunnel with an undersea segment ?

text: List of longest tunnels
World 's longest tunnels ( in use )

This list of longest tunnels ranks tunnels that are at least 13 km (43,000 ft) long. Only continuous tunnels are included. Pipelines, even those that are buried, are excluded. The longest tunnels have been constructed for water distribution, followed by tunnels for railways.
Type Name Location Length Year Comment
Water supply Delaware Aqueduct New York State , United States 137,000 m ( 85.1 mi ) 1945 4.1 m wide . New York City 's main water supply tunnel , drilled through solid rock
Water supply Päijänne Water Tunnel Southern Finland , Finland 120,000 m ( 74.6 mi ) 1982 16 m cross section
Water supply Dahuofang Water Tunnel Liaoning Province , China 85,320 m ( 53.0 mi ) 2009 8 m in diameter ( 50m cross section )
Water supply Orange-Fish River Tunnel South Africa 82,800 m ( 51.4 mi ) 1972 Longest continuous enclosed aqueduct in the southern hemisphere ( 22.5 m cross section )
Water supply Bolmen Water Tunnel Kronoberg / Scania , Sweden 82,000 m ( 51.0 mi ) 1987 8 m cross section
Hydroelectric Neelum Jhelum HydroPower Tunnel Muzaffarabad , Azad J & K , Pakistan 68,000 m ( 42.3 mi ) 2017 Part of the 969 MW Neelum Jhelum Hydropower Plant
Wastewater Tunel Emisor Oriente Mexico City , Mexico 62,500 m ( 38.8 mi ) 2008-2014 Water management in Greater Mexico City . Longest wastewater tunnel
Metro Guangzhou Metro Line 3 Guangzhou , China 60,400 m ( 37.5 mi ) Excl . branch 2005-2010 Guangzhou Metro . Longest metro/rapid transit tunnel
Railway Twin Tube Gotthard Base Tunnel Central Swiss Alps , Switzerland 57,104 m ( 35.5 mi ) and 57,017 m ( 35.4 mi ) 2016 Longest railway tunnel ; by geodetic distance ( of 55.782 km ( 34.661 mi ) ) between the two portals , it is also the world 's longest transit tunnel . Total 151.84 km ( 94.35 mi ) of broken out tunnels through solid rocks
Metro Beijing Subway Line 10 Beijing , China 57,100 m ( 35.5 mi ) 2008-2012 Beijing Subway
Railway Single Tube Seikan Tunnel Tsugaru Strait , Japan 53,850 m ( 33.5 mi ) 1988 74 m ; longest railway tunnel until 2016 . Longest tunnel with an undersea section . Undersea section measures 23.3 kilometres ( 14.5 mi )
Water supply Želivka Water Tunnel Central Bohemian Region , Czech Republic 51,075 m ( 31.7 mi ) 1972 5 m
Railway Twin Tube Channel Tunnel English Channel , United Kingdom / France 50,450 m ( 31.3 mi ) 1994 Second longest railway tunnel until 2016 . Longest underwater section , longest international tunnel ( 2×45 m + 1×18 m )
Railway Single Tube Yulhyeon Tunnel Seoul Capital Area , South Korea 50,300 m ( 31.3 mi ) 2016 107 m
Water supply River Arpa - Lake Sevan tunnel Armenia ( at the time of construction Soviet Union ) 48,314 m ( 30.0 mi ) 1981 Longest tunnel built to restore ecological balance . Transfers water from Arpa River to the Lake Sevan to recover its level
Metro Seoul Subway : Line 5 Seoul , South Korea 47,600 m ( 29.6 mi ) 1995 Longest metro/rapid transit tunnel until Guangzhou Metro Line 3 extension opened in 2010
Water supply Pahang - Selangor Raw Water Transfer Project Pahang & Selangor , Malaysia 44,600 m ( 27.7 mi ) 2014 5.2 meters in diameter
Metro Mass Rapid Transit ( Singapore ) : Downtown Line Singapore , Singapore 43,700 m ( 27.2 mi ) 2017 Longest rapid transit line in South East Asia
Water supply # 1 Tunnel , Yellow River Diversion to Shanxi North Line Shanxi , China 43,670 m ( 27.1 mi ) 2011 See South-North Water Transfer Project
Water supply # 7 Tunnel , Yellow River Diversion to Shanxi South Line Shanxi , China 43,500 m ( 27.0 mi ) 2002 See South-North Water Transfer Project