Underwater tunnels are a miracle of modern engineering only made possible through the increasingly remarkable developments of mineral sciences, ever-elaborate tunnel boring machines and the conveyor belt infrastructure around them to move tonnes upon tonnes of earth and debris efficiently.

Much has been said about the potential for a transatlantic tunnel and the myriad of extreme impracticalities that make it effectively impossible using current materials and technology, but one aspect of that which is not discussed enough is the issue of depth.

Not counting particularly deep trenches, the average depth of the Atlantic Ocean is over 3600m, which brings with it a huge number of challenges for tunnel boring machines, atmospheric pressure and forces exerted onto the tunnel itself.

To see how far we have to go, it is important to understand how far underwater tunnel construction has evolved and progressed, and an excellent illustration of this is exploring some of the deepest undersea tunnels in recent years.

The First Hundred Metres

The first underwater tunnel in Europe to reach an underwater depth of over 100 metres was the Flekkeroy Tunnel in Norway, and the timing of its completion and its purpose provides a useful starting point for exploring undersea tunnels.

The island of Flekkeroy is one of the southernmost points of Norway, but before the development of a tunnel, the inhabitants of the island relied heavily on a ferry service to commute to the mainland.

Building a tunnel, therefore, made a lot of sense, and it proved that progressively deeper undersea tunnels were indeed possible.

An even greater accomplishment was the Channel Tunnel, which at the time of its completion in 1994 was the longest undersea tunnel in the world and the deepest in Europe. To this day it still holds the record for the longest underwater section.

The Channel Tunnel remains a useful benchmark, given that if the transatlantic tunnel was built at the same speed it would take nearly a thousand years to build.

This depth record in Europe would be broken the same year by the Hitra Tunnel, but the world record for deepest tunnel that it would also beat was set as early as 1988.

The Second Hundred Metres

The Seikan Tunnel is the second-longest tunnel in the world and is still the longest tunnel with any underwater section. It was also the first tunnel to reach under 200m in depth.

Its ambition, scale, cost and controversy rival the Channel Tunnel, even if it is over double the depth at its deepest level. 

The tunnel took over two decades to build, cost 12 times its initial budget and was described as a “useless long thing”, a “quagmire tunnel” and even as one of the “Three Great Follies” of the Showa Era.

The latter statement, one which extended to the Shinkansen as a whole, ultimately proved to be incorrect, as the stability, safety and weather-resistant transportation means that the Seikan Tunnel became a vital part of Hokkaido’s agricultural industry.

It does, however, highlight the issue of timing when it comes to tunnels and other grand transportation projects; because of how long tunnels take to construct, they have to aim at a moving target to some degree and predict how they will be used in the future.

It would ultimately be supplanted as the deepest tunnel in the world by the Hitra Tunnel in 1994, which reached a depth of 264m below sea level.

The Third Hundred Metres?

As of 2025, no undersea tunnel has reached a depth of 300m, although two have come very close and by 2033 this is likely to change.

The Eiksund Tunnel in 2008 was, when it was completed, the deepest undersea tunnel of any kind, with a depth of 287m. 

By the 2000s, the concept of using tunnels to link the various fjords and islands to mainland Norway was well-established, and the Eiksund Tunnel was part of a complex of tunnels and bridges to connect over 40,000 people to the rest of Norway.

This record was ultimately broken by the Ryfylke Tunnel, which became in 2019 the deepest tunnel in the world at 292m below sea level, and as part of the Ryfast project, it became the longest undersea road tunnel in the world.

However, whilst the 300m milestone has not been beaten yet, not only will the proposed Rogaland Fixed Link reach well under 300m, but at a maximum depth of 392m will come close to 400m below sea level.

Even after nearly half a century, this is still a tenth of the depth required to build a tunnel under the Atlantic, and so it will require even greater and more exponential tunnel development to accomplish this in any of our lifetimes.