Weight is not what makes an object hard to move across water.
Displacement is. A ship of 80,000 tons is moved by tugs every day of the week, and nobody finds it remarkable.
So the interesting question about a tunnel section is not really how heavy the thing is.
It is how much of it is hollow.
Get that ratio right and an object this size behaves like a barge.
Get it wrong and it sits on the bottom of the basin forever.
The box is built to float by a very small margin
Each section is a hollow reinforced concrete shell with steel bulkheads welded across both ends.
Sealed like that, the air inside it does the work. Average the concrete and the void together and the whole object comes out just a shade lighter than the water it displaces.
So it floats, but barely. There is very little freeboard, and most of the section is under the surface even before anybody wants it there.
Tugs then have an easy job in principle, since they are fighting drag rather than lifting anything.
Sinking it is the deliberate part. Ballast tanks inside the section take on water until the balance tips, and it settles onto a prepared gravel bed while survey gear keeps it lined up.
Then comes the elegant bit. A rubber gasket on the front face touches the section already in place, and the water trapped in the gap between the two bulkheads is pumped out.
The sea does the rest, pressing on the far end with thousands of tons and squeezing the joint shut by itself.
What is being cast and where
The factory sits on the shore at Rødbyhavn on Lolland, with a basin cut into the coast so finished sections can be floated straight out.
A standard section runs 712 feet long and 138 feet wide, weighing about 81,000 tons.
Inside that cross section there are five tubes: two carrying a motorway lane pair each, two carrying electrified railway track, and a small service gallery between them.
Eighty nine sections in total will be laid end to end along the seabed.
Seventy nine of those are standard. The other ten are special sections, deeper and fitted with the plant rooms the tunnel needs.
The finished link runs 11 miles under the Baltic, which is what puts it ahead of every other immersed tunnel.
The first one went down this spring
The first section was floated out of the basin and immersed off Rødbyhavn on May 4 of 2026.
Five tugs moved it, working with a purpose built immersion vessel, and about 4,500 tons of extra ballast were added to bring it down.
Construction began in 2020 and the crossing is due to open in the middle of 2029.
The budget is around 8 billion dollars, with a substantial contribution from the European Union and the rest carried by Denmark.
The trip it replaces is a 45 minute ferry crossing. By car the tunnel takes ten minutes, and by train it takes seven.
The railway is wired for 125 miles an hour, which matters more for freight than for holiday traffic.
Three things worth stating carefully
The weight figure applies to a standard section. The ten special ones are a different shape and a different mass, so a single number for all 89 is a simplification.
The record claim needs its qualifier too. This is the longest immersed tunnel, which is a specific category and not the same as the longest undersea tunnel or the longest rail tunnel.
Bored tunnels beat it comfortably on raw length.
And the schedule has moved more than once. A crossing first talked about in the previous century has had its opening date pushed repeatedly, which is normal for work of this size, in the way that timelines around a major crossing or a record structure almost always slip somewhere.
The engineering itself is documented in unusual detail, as the institution’s project file sets out.
Why nobody simply bored it
A boring machine would have looked like the obvious choice a decade ago.
The seabed here argues against it. The strata are soft clay and till, the water is shallow, and a bored tube would have to dive well below the seabed and then climb back out on both sides.
Those approach ramps get very long indeed, and on flat farmland they eat up the land the crossing was meant to serve.
An immersed tunnel sits in a dredged trench instead, barely below the seabed, so the ramps stay short, as the operator’s own account of the first immersion describes.
The concrete boxes are not the clever part of this.
The trench under them is.
