[Science Scope] From Tunneling Through the Sea to Floating "Roads" Beneath the Waves

Thames River Mud, Seikan's High-Pressure Waters, Geoga's Immersed Tunnel, Norwegian Fjords...
And the Future of the 16 Trillion Won Jeju Submerged Floating Tunnel

Humanity's quest to cross the sea has always been a fierce battle against nature. It began with launching boats to overcome rough waves, progressed to building steel bridges over deep straits, and ultimately succeeded in tunneling through solid bedrock under the sea.

However, human imagination does not stop there. The next step is the Submerged Floating Tunnel (SFT), which floats a passage at mid-depth in the sea instead of digging along the seabed.

Although it may sound like science fiction, this technology did not emerge overnight. An idea that began in 1843 in the mud beneath the Thames River in London has accumulated over 200 years and is now aiming at the sea beneath Jeju.

London's Thames River: The Birth of the "Shield" Amid Collapsing Mud

In 1843, London, England, tunneling beneath the Thames was a challenge that bordered on madness. The riverbed was not solid bedrock but layers of mud and sand saturated with water. Even a small excavation would cause river water to pour in from above, instantly turning the worksite into an underwater grave.

In fact, during construction, river water flooded the work sections multiple times, causing repeated accidents where workers had to evacuate in a hurry.

The engineer who solved this problem was Marc Brunel. He devised the "iron shield," which was then implemented on site. Unlike today’s TBMs, the shield was not a self-propelling machine but a massive iron structure divided into multiple small work compartments. Each compartment held one worker, who would shovel out a bit of soil; then, the entire shield would be pushed forward by a few centimeters using hydraulic jacks and screw mechanisms.

Conceptual diagram of the Thames Tunnel's "Shield" construction method designed by Marc Brunel. It is a method of simultaneously excavating soil in multiple work compartments and building walls from behind while advancing forward. Provided by the Brunel Museum

Immediately behind, other workers would build rounded brick walls to create the permanent tunnel lining. In other words, while soil was being excavated at the front, walls were being built at the rear simultaneously, and the entire structure advanced gradually.

The key was speed. Because the Thames mud could not support itself, the rates of excavation and wall-building had to be nearly perfectly synchronized to prevent collapse. The Thames Tunnel did more than simply create a passage; it was humanity’s first technological answer to the question of "how to withstand collapsing weak ground."

Seikan Tunnel: Japan's Determination Piercing Seawater and Volcanic Bedrock

The next technological challenge arose in Japan. The Seikan Tunnel connecting Honshu and Hokkaido is a world-class undersea railway tunnel, stretching 53.85 kilometers, with 23.3 kilometers beneath the sea.

For Japanese engineers, the greatest difficulties were not the tunnel's length, but the high-pressure seawater and volcanic bedrock. Even a small fissure in the rock would lead to torrents of seawater under immense pressure, and during construction, massive flooding and collapse accidents occurred repeatedly.

Seikan Tunnel Overview Diagram. Seikan Tunnel Structural Overview. A method of excavating the main tunnel after first finding water paths through a preceding smaller pilot tunnel for drainage and sealing. Provided by JR Hokkaido.

Japan’s solution was not to excavate the main tunnel immediately. Instead, they first dug a smaller exploratory pilot tunnel to investigate where water might leak and where the bedrock was weak. This pilot tunnel was more than just a survey passage; it served as a giant drainage pipe and a risk detection sensor.

Wherever water ingress was detected, cement milk was injected into the gaps to block the water flow, and weak bedrock was reinforced with steel pipes and rock bolts. Simply put, it was like locating small blood vessels to stop bleeding before performing major surgery.

Thanks to this method, Japan was able to complete one of the world’s longest undersea tunnels after 24 years of construction. The Seikan Tunnel is an iconic example of overcoming undersea pressure and geological uncertainty through the "pilot tunnel" concept.

Korea’s Present: The Geoga Bridge Immersed Tunnel—A Road Sunk Beneath the Sea

For Korean readers, the most intuitive example is the immersed tunnel of the Geoga Bridge. Linking Busan and Geoje, the Geoga Bridge is Korea’s first immersed tunnel. Massive concrete sections hundreds of meters long were constructed at a land dock, floated to the site, and then sunk one by one into a pre-excavated trench on the seabed for assembly.

The most challenging part of construction was fitting the tunnel sections precisely into the seabed trench. Even slight currents could shift the structures by several centimeters, causing joint failure.

A submerged tunnel hull approximately 180 meters long, manufactured at the onshore dock. It is then moved offshore and submerged underwater to be connected. Provided by Daewoo E&C.

To solve this, GPS-based positioning, underwater acoustic systems, and laser alignment equipment were all deployed simultaneously. Guide frames were pre-installed on the seabed, and temporary buoyancy tanks were attached to the tunnel sections, allowing for subtle control of the sinking speed.

During the final joining stage, internal pressure was adjusted to tightly press rubber gaskets between the sections, and water was removed from inside, creating a near-vacuum that drew the sections together. In short, the Geoga immersed tunnel is not just a "road sunk into the sea," but the result of a marine precision operation involving alignment, buoyancy control, and pressure-sealed joining.

The next step is the technology of suspending a tunnel within the sea, not sinking it to the seabed—namely, the Submerged Floating Tunnel (SFT).

Norwegian Fjords: Choosing the Depths Rather Than the Seabed

Conceptual diagram of the submerged floating tunnel (SFT) under review in the fjord section of Norway. Vehicles move inside the submerged floating tunnel below the water surface, while ships pass above in the upper sea area. Provided by the Norwegian Public Roads Administration (NPRA).

Norway took a completely different approach. The Bjornafjorden and Romsdalsfjorden in western Norway are over 1,000 meters deep. Excavating a seabed tunnel like the Seikan Tunnel is impractical due to the depth, and the length required for bridge spans between piers is unrealistic.

Therefore, the Norwegian Public Roads Administration is seriously considering the SFT for the E39 project, which aims to connect the entire route without ferries. The key is to float the tunnel at a depth of 20 to 30 meters below the water surface, rather than on the seabed.

Representative structural concept diagram of the Subsea Floating Tunnel (SFT). It shows the method of fixing with subsea cables (a) and the method of controlling buoyancy with pontoon structures above the water surface (b). Provided by MDPI

The structural principle is surprisingly intuitive. The inside of the tunnel is hollow for vehicles and trains to pass through, while the outer shell is made of thick concrete and steel, making the entire structure slightly lighter than water. As a result, the structure naturally has upward buoyancy.

This buoyant force is counteracted by anchor cables fixed to the seabed, holding the tunnel in balance. When waves shake the structure from above, the cable tension immediately responds, reducing the amplitude. Some designs connect to pontoons on the water surface to further reduce vertical movement. In simple terms, the tunnel is stabilized in the sea by a tug-of-war between the upward buoyancy and the downward tension of the cables.

Kwak Jongwon, Senior Research Fellow at the Korea Institute of Civil Engineering and Building Technology, explained, "In civil engineering, there are very few things that are technically impossible. The SFT's concept and component technologies are already at a level where implementation is possible." However, he emphasized, "The main challenge is not the technology itself, but economic feasibility, safety, and, above all, public acceptance."

Overview diagram of the multi-layer underwater tunnel structure proposed by Pureundeul, which holds a registered patent for underwater floating tunnel structure and construction methods. It includes roads, railways, and pedestrian paths, with some sections featuring underwater and marine observatories for tourism purposes. Provided by Pureundeul

Jeju Submerged Floating Tunnel: A 16.8 Trillion Won Challenge and Time Stopped

This technology has now become a real challenge for Korea. The Mokpo-Jeju J-Route spans a total length of 167 kilometers, with about 73 kilometers undersea. The project cost is estimated at approximately 16.8 trillion won.

In fact, this discussion was the hottest topic in the early 2010s. The Korea Institute of Ocean Science and Technology (KIOST) and some civil and marine infrastructure researchers conducted studies at the basic design level. At that time, the SFT was widely seen as the future transportation network connecting Jeju.

However, discussions have essentially stalled since then. The biggest reason was the difficulty in securing economic feasibility (B/C ratio—Benefit/Cost ratio). Given that aviation demand was already high, questions arose over whether the massive initial investment could be recovered. The possibility of including Chuja Island as a stopover and projections for logistics demand were also variables. In addition, shifting national SOC priorities and failure to move past the preliminary feasibility stage contributed to the discontinuation of follow-up discussions.

Senior Research Fellow Kwak stated, "Costs can be addressed with technological advances, but psychological anxiety about safety and achieving social consensus are much bigger barriers. The issue of connecting Jeju to the mainland is less about technology and more about policy decisions."

Seo Seungil, Principal Researcher at the Korea Railroad Research Institute, offered a more concrete outlook on technical feasibility. He explained, "The SFT allows for a nearly straight route, which shortens construction time and improves economic efficiency. Combining subsea tunnels, immersed tunnels, and floating tunnels could provide a practical solution for long-distance undersea routes."

Recently, as discussions on alternative transportation networks to aviation, Jeju logistics stability, and national balanced development have intensified in the era of climate crisis, some experts are sensing a renewed atmosphere in which "SFT could be more realistic than conventional excavation methods."

Principal Researcher Seo noted, "Connecting Jeju under the sea involves differences in regional perspectives and technical preferences. Political consensus must come first."

"Driving While Watching Fish"... From Transportation to Tourism Platform

The reason SFT goes beyond simple civil engineering is its potential to transform the experience. Whereas conventional undersea tunnels are closed spaces with only dark concrete walls, the SFT could be expanded with observation windows or transparent reinforced materials, allowing travelers to directly view marine life.

Kang Seongsu, who holds a registered patent for underwater floating tunnel structures and construction methods, emphasized, "The underwater tunnel must now become more than a simple passageway—it should be a next-generation integrated platform combining underwater tourism, marine ecological observation, tidal power generation, and offshore wind power."

The road built 200 years ago by London’s workers, laying bricks in the Thames mud, is now becoming humanity’s new dream—traveling beneath the Jeju sea alongside schools of fish.

This content was produced with the assistance of AI translation services.