Undersea tunnel in the context of "Immersed tube"

The Gulf of Finland (Estonian: Soome laht; Finnish: Suomenlahti; Russian: Фи́нский зали́в, romanized: Finskiy zaliv; Swedish: Finska viken) is the easternmost arm of the Baltic Sea. It extends between Finland to the north and Estonia to the south, to Saint Petersburg—the second largest city of Russia—to the east, where the river Neva drains into it. Other major cities around the gulf include Helsinki and Tallinn. The eastern parts of the gulf belong to Russia, and some of Russia's most important oil harbors are located there, including Primorsk. As the seaway to Saint Petersburg, the gulf is of considerable strategic importance to Russia. Some of the environmental problems affecting the Baltic Sea are at their most pronounced in the shallow gulf. Proposals for an undersea Helsinki–Tallinn Tunnel through the gulf have been made.

Preveza (Greek: Πρέβεζα, pronounced [ˈpreveza]) is a city in the region of Epirus, northwestern Greece, located on the northern peninsula of the mouth of the Ambracian Gulf. It is the capital of the regional unit of Preveza, which is the southern part of the region of Epirus. The Aktio-Preveza Immersed Tunnel –the first, and so far only, undersea tunnel in Greece– was completed in 2002. The 1,570 m (5,150 ft) long immersed tunnel connects Preveza in the north, to Aktio of western Acarnania to the south. The ruins of the ancient city of Nicopolis lie 7 kilometres (4 miles) north of Preveza.

38°18′N 120°54′E / 38.3°N 120.9°E / 38.3; 120.9 (Bohai Strait)

The Bohai Strait Tunnel or Dalian-Yantai Tunnel is a proposed undersea tunnel construction project across the Bohai Strait to connect Dalian on the Liaodong Peninsula with Yantai on the Shandong Peninsula. The official name for the project is Bohai Strait Cross-Sea Corridor (Chinese: 渤海海峡跨海通道).

Riprap (in North American English), also known as rip rap, rip-rap, shot rock, rock armour (in British English) or rubble, is human-placed rock or other hard, heavy, unconsolidated material used to protect shoreline structures against scour and water, wave, or ice erosion. Riprap is used to armor shorelines, streambeds, bridge abutments, foundational infrastructure supports and other shoreline structures against erosion. Common rock types used include granite and modular concrete blocks. Rubble from building and paving demolition is sometimes used, as well as specifically designed structures called tetrapods or similar concrete blocks.Riprap is also used underwater to cap immersed tubes sunken on the seabed to be joined into an undersea tunnel.

The Helsinki–Tallinn Tunnel (also known as FinEst or Talsinki tunnel) is a proposed undersea tunnel that would span the Gulf of Finland and connect the Finnish and Estonian capitals by train. The tunnel's length would depend on the route taken: the shortest distance across would have a submarine length of 80 kilometres (50 mi), which would make it 40% longer than the current longest railway tunnel in the world, the 57 km (40 mi)-long Gotthard Base Tunnel in Switzerland.

During the 2010s, it was estimated that the tunnel, if constructed, would cost €9–13 billion and could open in the 2030s if approved. In 2013, the European Union approved €3.1 million in funding for feasibility studies. A 2015 pre-feasibility study proposed trains traveling with a top speed of 250 km/h (155 mph). On 8 February 2024, Finnish Minister of Transport and Communications Lulu Ranne stated in an interview for Estonian newspaper Postimees that the tunnel is "unrealistic" and not on the government's agenda, with the project remaining on hold unless the European Union decides to allocate additional funding for it.

The Aktio–Preveza Undersea Tunnel is an undersea road tunnel across the mouth of the Ambracian Gulf in western Greece. It links Epirus and the city of Preveza on the north shore of the gulf with the cape of Aktio (Actium) in Aetolia-Acarnania, in Central Greece. Completed in 2002, the tunnel is an important piece of infrastructure in an underdeveloped region, and greatly shortens the travel distance between the two sides of the gulf, which had been only possible by ferry. It is the only undersea tunnel in Greece.

The Hong Kong–Zhuhai–Macau Bridge (HZMB) is a 55-kilometre (34 mi) bridge–tunnel system consisting of a series of three cable-stayed bridges, an undersea tunnel, and four artificial islands. It is both the longest sea crossing and the longest open-sea fixed link in the world. The HZMB spans the Lingding and Jiuzhou channels, connecting Hong Kong and Macau with Zhuhai—a major city on the Pearl River Delta in China.

The HZM Bridge was designed to last for 120 years and cost ¥127 billion (US$18.8 billion) to build. The cost of constructing the Main Bridge was estimated at ¥51.1 billion (US$7.56 billion) funded by bank loans and shared among the governments of mainland China, Hong Kong and Macau.