The world’s longest underwater high-speed train is now in progress, set to link two continents beneath the sea

From a distance, the sea looks calm on a grey, windy morning off the coast of northern China. It’s a construction site the size of a small city when you get up close. Barges hum, cranes swing slowly over the waves, and somewhere under all that steel and spray, crews are getting the ground ready for a tunnel that sounds like something out of science fiction: a high-speed rail line that will go under the ocean and come back up on another continent.

Engineers yell over the noise and point to screens that show the seafloor like an X-ray. As a survey drone rises into the air, a diver climbs back on deck with a dripping helmet.

It’s dirty, cold, and not very glamorous.

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But these are the first real steps toward what could be the longest underwater high-speed train in the world.

A train that will go under the sea and change how we think about the world.

The silent race to connect continents under the sea

For years, the thought of taking a fast train from Asia to Europe without ever seeing the sky seemed like a dream that would never come true. It’s now written down on engineering plans and government bids. The most ambitious plan on the table is to connect mainland China to Europe through a series of tunnels and bridges. The most impressive part of this plan would run under the Bohai Strait and eventually connect through Central Asia to the West.

This isn’t just one tunnel, like the one that goes between France and the UK. It’s a brand-new spine, buried under rock and seabed. Trains will go as fast as planes while passengers sit and scroll through their phones.

The Bohai Strait section, which is already being built, is the heart of this dream. There are about 125 kilometres of underwater and underground structures, including seabed tunnels and artificial islands, that are meant to hold high-speed trains that can go up to 350 kilometres per hour.

Engineers say they can cut the time it takes to get from one megacity port to another from hours to less than one. Local fishermen talk about whether or not the fish will change.

People are more interested in practical things on land. How much more will a ticket cost than a flight? How will it feel to go full speed for almost an hour with the sea pressing down on you hundreds of meters above your head?For political reasons, it needs something even harder: long-term cooperation between countries that don’t always agree with each other.

But the reasoning behind it is strong. Planes are fast, but they pollute the air and are affected by fuel prices. Ships carry a lot of stuff, but they move slowly across the map. In the middle is a high-speed train that goes under the sea. It’s fast, electric, and never stops. When that equation starts to make sense, big projects seem to have to happen.

How do you even make a bullet train that goes under the ocean?

Engineers start on land when they want to build a train line under the sea. They dig test shafts, look at the rock, and make maps of every crack. Then you have to choose between digging a deep hole through solid rock or putting together pre-made tunnel pieces in a trench on the seabed.

The new mega-projects are using a little bit of both. Huge tunnel boring machines, like metal worms that are as long as a football pitch, move forward from each shore. At the same time, ships drop hollow concrete pieces into dredged channels and seal them together like Lego blocks in the water.

A secret highway is being built where fish used to swim, piece by piece.

This is where things start to get real and breakable. Weather can stop work for days. If a segment is off by just a few centimetres, it could lead to weeks of redesign. One experienced project manager said it was like “trying to thread a needle while the table is moving and someone keeps changing the light bulb above you.”

The Channel Tunnel between France and the UK used to seem crazy, but now it seems almost normal next to these new plans. The tunnel is about 50 kilometres long. The Bohai crossing alone wants to do more than double that, and the full Asia–Europe corridor makes it even bigger.

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And this time, trains won’t just crawl across. They’ll run fast.

The magic only works for the passenger if the trip is boring in the best way. No leaks, no rattles, and you don’t feel like you’re under hundreds of meters of water. That calls for obsessive design.

Signals must work perfectly through rock and water. Even in the most remote areas, emergency exits should be easy to get to in a matter of minutes. Trains need closed cabins that can handle changes in pressure without waking up kids.

To be honest, no one really looks at all the safety diagrams in the carriage.

So the real work is to make everything so strong that people never have to.

The emotional shock of making a planet smaller

Transport planners use a simple way to measure impact: they look at “time distance” instead of kilometres. There are two cities that seem like they are worlds apart because they are eight hours and three connections away from each other. Then picture a single high-speed train that runs under the sea and connects them in three and a half hours, from door to door.

Job markets in other countries suddenly feel like they’re in your own country. It becomes normal to visit family members who live abroad on the weekend. Students can choose a university on a different continent without having to think about crossing an ocean.
*The map in your head changes without you asking it to.

Of course, big promises come with big risks. We’ve all been there: a shiny new queue opens, and then the tickets turn out to be for executives, not regular families. The story changes if the world’s longest underwater high-speed train becomes a luxury toy.

There is also the quieter, emotional fear: some people just don’t like the idea of being trapped in a metal tube far below the waves. Claustrophobia doesn’t care about blueprints.

The people who work on these projects know it. They talk about lighting, colours, peaceful rooms, screens that show real-time information, and even soundscapes that have been carefully chosen. Little things that make a harsh environment feel like just another commute.

One psychologist who works with people who have to go through long tunnels says, “It’s not the sea above them that scares them.” “It’s the feeling of being stuck and having no control.” Even if the physics are set in stone, our job is to create spaces and routines that give people back a sense of choice.

• Clear signs and soft lightingMakes you feel less like you’re buried underground and naturally leads the eye.
• Cabins that are quiet and stableLessens the rattling and noise that can make people anxious, especially on long stretches of water.
• Clear, honest informationLive maps, updates on your journey, and clear explanations during any delays help calm the “what’s going on?” panic.
• Ticket rules that seem fair Early discounts, passes, and social fares decide who really owns this new link.
• Integration across continentsTransfers to local trains, subways, and buses that are easy to use make a megaproject fit into everyday life.

When continents are like neighbourhoods

The strangest part of this story is how quickly that line would change what we think of as “far away.” We’ve already seen this happen with high-speed rail in the US. A 300-kilometer trip drops below that invisible mental barrier of “too much effort,” so cities that used to seem far away now share the same job market, dating apps, and cultural events.

Take that logic and apply it to other continents. A student from northern China is taking an overnight train under the sea to do an internship in Western Europe. A small exporter who doesn’t want to deal with complicated air freight and instead sends goods by high-speed rail, which gets there in days instead of weeks. A doctor who travels across Eurasia every month to run a specialised clinic.