Metro rail and rail tunnels in Asia and Europe have reached 37.5 miles long.
The Channel tunnels is 31.3 miles long and has the longest underwater section.
China is already building the Yunnan–Guizhou Water Tunnel which will be 410 miles long (661 kilometers). It is to be completed by 2025.
There have been several studies for a tunnel to cross the Strait of Gibraltar. The proposed rail Gibraltar tunnel’s length is 40 kilometres (25 mi), 300 metres (980 ft) deep, and its construction would take 15 years. An earlier plan was to link the two continents via the narrowest part of the strait, but this idea was dismissed as the tunnel would be 900 metres (3,000 ft) below sea level. For comparison, the deepest undersea tunnel, the Eiksund Tunnel, is 287 metres (942 ft) below sea level.
The construction of terminals, such as the Eurotunnel Folkestone Terminal and Eurotunnel Calais Terminal at either end of the Channel Tunnel, might be required for trans-shipment of road vehicles.
Helsinki to Tallinn Tunnel
The Helsinki to Tallinn Tunnel is a proposed rail undersea tunnel that would span the Gulf of Finland and connect the Finnish and Estonian capitals. The shortest distance across would have a submarine length of 50 kilometres (30 mi), making it the longest undersea tunnel in the world (both the Channel Tunnel and Seikan Tunnel, whilst longer, have less length undersea). The tunnel, if constructed, is estimated to cost €9–13 billion and would open sometime after 2030. The European Union has approved €3.1 million in funding for feasibility studies.
The economic benefits would be significant, both in terms of increased connections and economic integration between the two cities but also in a wider context of convenient passenger train connections between Southern Finland and the Baltic states, and a fixed link for freight from across Finland on to the Rail Baltica, thus providing a rail freight connection with the rest of Europe.
Bohai Strait-Crossing Tunnel
The Bohai Strait tunnel would be 123 kilometres (76 mi) long, 90 kilometres (56 mi) of it under water. This would exceed the combined lengths of the two longest undersea tunnels on Earth, the Seikan Tunnel and the Channel tunnel. The project is estimated to cost 260 billion yuan (US$41 billion). In August 2014, it was reported that work is likely to commence during the 13th Five-year plan and construction would take ten years to complete. As of March 2018, the central government now wants to get construction underway on this bay connection ‘as soon as possible’.
Passenger vehicles would be loaded onto rail carriages and transported at up to 250km/h, shortening driving time between Dalian and Yantai to an estimated 40 minutes. Currently ferries between the two cities, which are about 170 kilometres apart, require eight hours to make a single trip. The investment of 260 billion yuan was projected to break even within 12 years. Daily traffic flow between Dalian and Yantai was expected to increase to more than 100,000 vehicles by 2015.
The Bohai Strait-Crossing Tunnel will be located on the shortest corridor from Northeast China to East China and South China. Laotieshan Channel is an important sea navigation channel, so a bridge cannot be built across it. The width of Laotieshan Channel reaches 50 km, so a highway tunnel cannot be built because the problems of ventilation, disaster prevention, and emergency rescue cannot be solved at present. Therefore, a fixed railway link may be preferentially considered in the short term, although the route of the fixed highway link should be preserved.
The route of the Bohai Strait-Crossing Tunnel crosses many islands; this allows the long tunnel to be divided into several sections, is favorable for ventilation during construction and operation, and is favorable for emergency rescue. Considering the requirements of emergency rescue and emergency evacuation, the tunnel may consist of three parallel tubes, with the central tube serving as an emergency rescue passage. Due to the thick cover of the tunnel, the tunnel will be mostly located in bedrock. The surrounding rock of the tunnel will mostly be granite. The tunnel may be constructed by tunnel-boring machines (TBMs) with about 10 meters in diameter, assisted by drill and blast. However, the 25 km long single-direction boring of a TBM under marine water is a major challenge.
Taiwan Strait-Crossing Tunnel
The Taiwan Strait-Crossing Tunnel needs to cross the Taiwan Strait over a width of more than 100 km. The water of the Taiwan Strait is about 85 m deep, and the ground below the water is mainly composed of interbed consisting of Tertiary sandstone and shale of different thicknesses. The thickness of the sub-horizontal sandstone and shale ranges from 200 m to 300 m. At present, the optimal option may be to build a railway tunnel, which may be constructed by open TBM assisted by the drill-and-blast method.
In 2017, Chinese Academy of Engineering completed a detailed design for an 84 mile long Tunnel to Taiwan. The Academy is the largest central governmental advisory body on infrastructural construction.
Beijing included a cross-strait high-speed rail network in its then-new five-year plan. But a lot of work has been need to get a viable technical design.
There is also the political challenge. Taiwan does not want the tunnel and a majority do not want reunification with China.
The China-Taiwan tunnel would start from Pingtan, a pilot free-trade zone area set up by Beijing in Fujian province in 2013 to boost trade with Taiwan. Digging would go down nearly 200 meters (218 yards). It would cut through complex layers of rock, including extremely hard granite and avoid at least two major earthquake faults and return to the surface in Hsinchu, a coastal city near Taipei.
It would be 135 kilometers long. This is three and a half times longer than the 37.9 km Channel Tunnel between Britain and France.
The Channel Tunnel – nicknamed the “Chunnel” – took six years to build and cost the equivalent of 12 billion euros (US$13.99 billion) today.
The Taiwan-China tunnels would have three main tunnels and each would be nearly a third larger than their European counterparts. Two main tunnels would each be 10 meters in diameter.
Two main passages would be used by trains running in opposite directions. There would be a smaller service tunnel that would contain power lines, communication cables and emergency exits.
Their broader design would allow trains to travel faster and carry bulkier cargos.
Two artificial islands would have to be created in the middle of the Taiwan Strait for air treatment stations that would pipe fresh air into the tunnel.
China will spend almost US$2 billion (12 billion yuan) on a 6.8 mile (11 kilometer) long highway tunnel.
The Pingtan-Fuqing passageway, running more than 11 kilometers under the seabed, would beat the Tokyo Bay Aqua Line as the world’s longest undersea road tunnel. The Chinese road tunnel will be wide enough for three heavy cargo trucks to drive side by side in each direction.
There is a 26 million yuan feasibility assessment for building a road tunnel from Pingtan to Fuqing.