science

Underwater tunnel

Underwater tunnel is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Underwater tunnel rather than just read about it. In short: An underwater tunnel is a tunnel which is partly or wholly constructed under the sea or a river. They are often used where building a bridge or operating a ferry link is unviable, or to provide competition or relief for existing bridges or ferry links.

Key takeaways

Underwater tunnel belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Underwater tunnel to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Underwater tunnel from memory before moving on to harder problems.

Reference excerpt

An underwater tunnel is a tunnel which is partly or wholly constructed under the sea or a river. They are often used where building a bridge or operating a ferry link is unviable, or to provide competition or relief for existing bridges or ferry links. While short tunnels are often road tunnels which may admit motorized traffic, unmotorized traffic or both, concerns with ventilation lead to the longest tunnels (such as the Channel Tunnel or the Seikan Tunnel) being electrified rail tunnels.

Types of tunnel Various methods are used to construct underwater tunnels, including an immersed tube and a submerged floating tunnel. The immersed tube method involves steel tube segments that are positioned in a trench in the sea floor and joined together. The trench is then covered and the water pumped from the tunnel. Submerged floating tunnels use the law of buoyancy to remain submerged, with the tunnel attached to the sea bed by columns or tethers, or hung from pontoons on the surface.

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

See also Immersed tube tunnel Intercontinental and transoceanic fixed links Shark tunnel

References

Worked examples

Example 1 — a first encounter with Underwater tunnel

Start with the simplest possible case. Write down what Underwater tunnel claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Underwater tunnel before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Underwater tunnel ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Underwater tunnel

In research: Underwater tunnel appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Underwater tunnel in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Underwater tunnel is common in secondary-school and first-year university syllabi. It links to neighbouring topics Coastal construction, Undersea tunnels, so understanding it makes those chapters shorter.
In everyday life: Look for Underwater tunnel outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Underwater tunnel in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Underwater tunnel means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Underwater tunnel out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Underwater tunnel in simple terms?

Why does Underwater tunnel matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Underwater tunnel?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Underwater tunnel.