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Currently under construction, the Fehmarn Belt Tunnel linking Germany with Denmark is set to become the world's longest pre-fabricated undersea road and rail tunnel when it's completed in 2029. But another subsea tunnel also being built, this one in China and scheduled for completion within a decade or so, looks likely to break that record. Longer, deeper, and with more capacity for traffic is what subsea tunnels are all about in the 21st century. But how exactly have these pioneering examples of civil engineering evolved over the last 180 years or so, and what underwater tunnel is presently the longest and deepest of them all?
Click through this gallery and submerge yourself in the history of undersea tunnels.
Work is progressing well on a record-breaking tunnel being built under the Baltic Sea. Running for 11 miles (18 km), the Fehmarn Belt Tunnel, or Fehmarnbelt, will be the world's longest pre-fabricated road and rail tunnel.
The undersea transport link between Germany and Denmark is a remarkable feat of engineering. Rather than burrowing through bedrock beneath the seabed, construction will see segments of the tunnel placed on top of the seafloor, and then joined together. The project is scheduled for completion by 2029.
The first tunnel known to have been constructed successfully underwater was the Thames Tunnel. Set under London's River Thames, it connected Wapping and Rotherhithe and was completed as a foot tunnel in 1843.
An astonishing accomplishment for the era, it was converted to a railway tunnel for the East London Railway in the 1860s. Sir Marc Isambard Brunel designed the tunnel, and his son, Isambard Kingdom Brunel—one of the greatest figures of the Industrial Revolution— was site engineer.
The Mersey Railway Tunnel opened in 1886 and is the oldest underwater rail tunnel in the world, crossing the River Mersey in Liverpool, England. It's one of the three Mersey tunnels, the other two being the Queensway Tunnel, opened in 1934, and the Kingsway Tunnel, which was inaugurated in 1971.
Another historic underwater rail tunnel, the Severn Tunnel runs under the estuary of the River Severn in England. Work began in 1873 (pictured). When it was completed, the tunnel, at 4.3 miles (7 km), was the longest of its kind until 1987—when Japan's Seikan Tunnel was opened.
Originally serving as a single bore, the Blackwall Tunnel is a pair of road tunnels underneath the River Thames in east London. Building it was difficult. The tunnel had to be excavated through sand, and in places ran less than six feet (1.8 m) beneath the river bed. When the tunnel opened fully in 1897, it was the longest underwater tunnel in the world. A second parallel tunnel was opened in 1967 to ease traffic congestion.
The Elbe Tunnel represents a pioneering example of civil engineering and was applauded as a technical sensation when it opened in 1911. The 1,398 ft (426 m) tunnel is constructed of two 20 feet (6 m) diameter tubes connecting the German city of Hamburg with the docks and shipyards on the south side of the Elbe River. The tunnel is accessible 24 hours for pedestrians and bicycles; however, since 2023 it has been closed to all vehicular traffic.
At the time of its opening in 1927, the Holland Tunnel was the longest continuous underwater tunnel for vehicular traffic in the world. The tunnel is set under the Hudson River that connects Hudson Square and Lower Manhattan in New York City in the east to Jersey City, New Jersey, in the west.
The tunnel reduced traffic on Hudson River ferries by 50%. The tradeoff, however, was severe traffic congestion as automobiles queued to enter the bridge after passing toll booths at the New York entrance on Broome Street.
Connecting Ontario, Canada, to Michigan in the United States under the Detroit River, the Detroit-Windsor Tunnel is the second-busiest crossing between the United States and Canada, the first being the Ambassador Bridge. The tunnel opened in 1928 and today serves approximately 13,000 vehicles daily.
The Tongyeong Undersea Tunnel has the distinction of being the first undersea tunnel in Asia, completed in 1932 when Korea was under Japanese rule. Originally designed for motor cars, the tunnel today only carries pedestrians.
The first tunnel under the River Mersey in Liverpool was designed for the previously mentioned Mersey Railway in 1886. In 1934 as an answer to the long queues of cars and trucks lining up at the Mersey ferry terminal, Queensway Tunnel was opened.
Queensway Tunnel contains a single carriageway of four lanes, two in each direction, though when it opened it featured just two lanes, with one in each direction (pictured).
Kanmon Railway Tunnel is one of three tunnels underneath the Kanmon Straits in Japan. Completed in 1942, it was the first undersea tunnel in the country, connecting the islands of Honshu and Kyushu.
The surrender of Japan in August 1945 spared the destruction by Allied forces of the railway tunnel. The Kanmon Roadway Tunnel opened in 1958, along with a separate pedestrian passage (pictured), built directly underneath the roadway tunnel.
The Lincoln Tunnel is in fact set of road tunnels built in three stages, crossing the Hudson River between Manhattan and New Jersey. The south tube, the final tunnel to be completed, was opened in 1957. This picture was taken as one of the first cars to test the runway passed through.
The tunnel's three tubes, operated by the Port Authority of New York and New Jersey, comprise six traffic lanes in total and carry a combined total of some 120,000 vehicles daily, making it the busiest vehicular tunnel in the world.
Cuba's Havana Tunnel was constructed during the Batista era, opening in 1958. The tunnel is set under Havana Bay and links Habana del Este with Havana Vieja.
The early 1960s saw the appearance of bridge-tunnel structures, and one of the first to be built was the Chesapeake Bay Bridge–Tunnel. It opened in 1964 to cross the mouth of Chesapeake Bay between Delmarva and Hampton Roads in Virginia. The tunnel section is one mile (1.6 km) in length.
The idea of an underwater rail tunnel traversing San Francisco Bay was put forward to city planners as early as 1872. A little over 100 years later, that far-fetched proposal became a reality with the opening of the Transbay Tube.
Construction was started on the tube in 1965 and completed in 1974. Built for Bay Area Rapid Transit, the Transbay Tube connects Oakland to San Francisco and is the longest underwater tunnel in North America, at 3.6 miles (5.8 km).
The Cross-Harbour Tunnel is the first tunnel in Hong Kong built underwater. Completed in 1972, It has since become one of the most congested roads in Hong Kong and the world, with 117,000 vehicles passing through it daily—nearly as many as the Lincoln Tunnel.
When it opened in 1983, the Vardø Tunnel became the first subsea tunnel in Norway. It was built under the Bussesundet Strait to connect the island of Vardøya to the village of Svartnes on the Varanger Peninsula on the mainland.
At 33 miles (53 km) in length, Japan's Seikan Tunnel is the world's longest tunnel with an undersea segment. It carries the Hokkaido Shinkansen, a Japanese high-speed rail line.
Construction on the Seikan Tunnel commenced in 1972, and was completed in 1988. The tunnel runs beneath the seabed of the Tsugaru Strait and connects the islands of Honshu and Hokkaido.
Completed and opened to traffic in 1992, the Sydney Harbour Tunnel provides a second vehicular crossing of Sydney Harbour to alleviate congestion on the Sydney Harbour Bridge. It's one of two transportation tunnels under the city's Port Jackson waters, the other being a set of rail tunnels for the Sydney Metro.
The Channel Tunnel is the only fixed link between the island of Great Britain and the European mainland. A prodigious industrial adventure, the project mobilized 12,000 engineers, technicians, and workers, who created the world's longest underwater tunnel over nearly 24 miles (38 km) from northern France to southern England.
The Tokyo Bay Aqua-Line is an expressway that connects the city of Kawasaki in Kanagawa Prefecture with the city of Kisarazu in Chiba Prefecture, and is largely made up of a bridge–tunnel combination across Tokyo Bay in Japan. An artificial island, Umihotaru (pictured), marks the transition between the bridge and tunnel segments.
It cost over US$1 billion to construct the Port Miami Tunnel. Built to alleviate cargo traffic in downtown Miami, the tunnel opened in 2014. The tunnel travels beneath Biscayne Bay, connecting the MacArthur Causeway on Watson Island with Port Miami on Dodge Island.
The Ryfast Tunnel actually consists of two subsea tunnels: Ryfylke Tunnel and Hundvåg Tunnel. Ryfast runs between the city of Stavanger, under a large fjord, and the area of Solbakk. In 2024, luxury carmaker Bentley set a new "underwater speed record" with the Continental GT Speed, running the car through Ryfylke Tunnel, currently the world's longest subsea road tunnel at 8.9 miles (14.4 km) long, and the deepest tunnel of any kind. The vehicle reached its top speed of 208 mph (335 km/h) in just 33 seconds, establishing an unofficial record for a car in a tunnel.
The Eysturoy Tunnel, or Eysturoyartunnilin, runs under the Tangafjørður Sound in the Faroe Islands, connecting the island of Streymoy to the island of Eysturoy. The tunnel is unique for including the world's first undersea roundabout in the middle of the network.
The first Qingdao Jiaozhou Bay Tunnel, which crosses underneath Jiaozhou Bay in China's Shandong Province, was completed in 2011. A second tunnel is currently under construction (pictured), which is set to become the world's longest subsea road tunnel, at 9.87 miles (15.89 km), to surpass Ryfylke Tunnel.
Sources: (GovHK) (Offshore Energy Magazine) (CNN) (Sacyr)
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LIFESTYLE Engineering
Currently under construction, the Fehmarn Belt Tunnel linking Germany with Denmark is set to become the world's longest pre-fabricated undersea road and rail tunnel when it's completed in 2029. But another subsea tunnel also being built, this one in China and scheduled for completion within a decade or so, looks likely to break that record. Longer, deeper, and with more capacity for traffic is what subsea tunnels are all about in the 21st century. But how exactly have these pioneering examples of civil engineering evolved over the last 180 years or so, and what underwater tunnel is presently the longest and deepest of them all?
Click through this gallery and submerge yourself in the history of undersea tunnels.