Underwater tunnel

Advantages

Compared with bridges

One such advantage would be that a tunnel would still allow shipping to pass. A low bridge would need an opening or swing bridge to allow shipping to pass, which can cause traffic congestion. Conversely, a higher bridge that does allow shipping may be unsightly and opposed by the public, and should it collapse, it may block shipping. Higher bridges can also be more expensive than lower ones. Bridges can also be closed due to harsh weather such as high winds.

Tunneling makes excavated soil available that can be used to create new land (see land reclamation). This was done with the rock excavated for the Channel Tunnel, which was used to create Samphire Hoe.

Compared with ferry links

As with bridges, albeit with more chance, ferry links may be closed by adverse weather (strong winds) or tides. Travelling through a tunnel is significantly quicker than travelling using a ferry link, as shown by the times for travelling through the Channel Tunnel (75–90 minutes for ferry and 21 minutes on the Eurostar). Ferries also usually use fossil fuels emitting greenhouse gases while most railway tunnels are electrified. In the Baltic Sea, one of the busiest areas for passenger ferries in the world, sea ice is a problem, causing seasonal disruption or requiring expensive ice-breaking ships. In the Øresund region the construction of the bridge-tunnel has been cited as enhancing regional integration and giving an economic boom not possible with the previous ferry links. Similar arguments are used by proponents of the Helsinki-Tallinn tunnel in the Talsinki region. There are various issues with the safety of both tunnels and ferries, in the case of tunnels, fire is a particular hazard with several fires having broken out in the Channel Tunnel. On the other hand, the free surface effect is a significant safety risk for RORO ferries as seen in the sinking of MS Estonia. Tunnels which exclude dangerous, combustible freights and the fuel or lithium-ion batteries carried aboard motorcars can significantly reduce fire risk.

Disadvantages

Compared with bridges

Tunnels can require higher costs of security and construction than bridges, but this depends on the clearance needed for ships and the water depth. This may mean that over short distances bridges may be preferred rather than tunnels (for example Dartford Crossing).

Compared with ferry links

As with bridges, ferry links are far cheaper to construct than tunnels, but not to operate. Also tunnels don't have the flexibility to be deployed over different routes as transport demand changes over time. Without the cost of a new ferry, the route over which a ferry provides transport can easily be changed. However, this flexibility can be a downside for customers who have come to rely on the ferry service only to see it abandoned. Fixed infrastructure such as bridges or tunnels represent a much more concrete commitment to sustained service.

List of notable examples

Proposed

Road

• Rogfast tunnel in Norway – construction having started in 2018, at 27 km length, 392 m depth, it will be the longest road tunnel and deepest undersea tunnel in the world.
• Karnaphuli Tunnel or Bangabandhu Sheikh Mujibur Rahman Tunnel in Bangladesh Tunnel is an underwater expressway tunnel in the port city of Chittagong, Bangladesh under the Karnaphuli river.
• Underwater Road Tunnel Salamina island-Perama - planned road tunnel in Attica, Greece. Currently at the second stage of the tender from which the concessionaire will be selected.
• India-Sri Lanka Sea Tunnel (proposed)
• Penang Undersea Tunnel in Malaysia – to open in 2025
• Western Harbour Tunnel in Sydney, New South Wales, Australia – to open in 2028
• Suðuroyartunnilin in the Faroe Islands – at least 25 km in length, it would connect the islands of Suðuroy and Skúgvoy to Sandoy, which is part of the fixed-link interconnected Faroese "mainland".
• Kross-Heimaey in Iceland - at 18 km length it would connect the island Heimaey to the mainland in South Iceland (geological studies and core sampling ongoing as of 2026)

Rail

• Bohai Strait tunnel in China between Dalian and Yantai (decided, construction to start 'as soon as possible'.)
• Helsinki to Tallinn Tunnel under the Gulf of Finland (proposed)
• Irish Sea Tunnel (suggested)
• Rio de Janeiro Metro Bay Tunnel (Line 3 – Rio de Janeiro-Niterói) (proposed)
• Fehmarn Belt Fixed Link between Denmark and Germany (decided, construction started in January 2021)
• Mumbai–Ahmedabad high-speed rail corridor of India (decided, construction start November 2018)
• Taiwan Strait Tunnel - if built would become the longest rail tunnel in the world. Engineering challenges and the unsolved political status of Taiwan make construction unlikely
• Strait of Gibraltar Tunnel - linking Gibraltar or the Spanish mainland to the African mainland. If built it would most likely become the deepest tunnel ever built