In Istanbul, commuter trains run 60 meters beneath the sea through the deepest immersed tunnel ever sunk, the first rail link joining Europe and Asia under the Bosphorus

In Istanbul, the train does something that costs hundreds of dollars at airports: it transports thousands of commuters from Europe to Asia in just minutes. No lines at the passport office, no lines at the boarding gates, and no turbulence, only steel tracks disappearing beneath the Bosphorus.

The Marmaray Tunnel is more than just a clever way to get around. It's listed in Guinness World Records as the first tunnel to connect two continents. It is submerged and is about 60 meters below sea level. It's one of the few infrastructure projects that is just as much a part of an engineering textbook as it is of a travel doc.

A commute unlike any other


In Istanbul, crossing the continent is like an every-Tuesday activity. The Marmaray Tunnel, which opened in October 2013, connects the European and Asian sides of Istanbul, under the Bosphorus Strait, the place that has separated and united civilizations for thousands of years.

The entire stretch is 13.5 kilometers and includes a 1.4-kilometer undersea section, confirming its status as a major underwater rail link. The time taken was once determined by traffic, weather, and ferry schedules, but is now defined by a few minutes. Trains take approximately 4 minutes to cross the underwater link, but the importance of the crossing is much greater than the length of time to cross.

Bridging history through modern engineering

Turkey's city of Istanbul is like a city of paradoxes. Its streets are inhabited by Roman, Byzantine, Ottoman and modern Turkish people. The Bosphorus has never just been a land divider; it has been a factor of commerce, conquest, migration and culture. Surprisingly, the plan to link both shores by train is even older than the car. Its historical proposal, first given by Sultan Abdülmecid I, goes back to the nineteenth century when the Ottomans were in power. But 21st-century engineering was required to make that vision a reality.

The building of the tunnel involved lowering and sinking prefabricated tunnel sections into place on the seabed, with a very high degree of accuracy, and then connecting them together underwater. Another inconvenient fact was taken into consideration by the engineers: Istanbul is located close to one of the world's most closely monitored fault zones, the North Anatolian Fault. The tunnel was designed to absorb seismic movement by not just being stiff and strong, but rather by being flexible and capable of absorbing the movement of the earthquake.

The sea gave up more than space

Mega projects often tend to re-sculpt landscapes. The excavation yielded unexpected historical discoveries for Marmaray. Digging near Yenikapı led to the discovery of one of the biggest collections of Byzantine shipwrecks ever located and thousands of artifacts which have changed historians' view of ancient Constantinople. The digging slowed down as discoveries were made.

Many were frustrated by the delays, but they also were a reminder that each ambitious infrastructure project is built on previous human stories. The ground refuses to give up so easily sometimes.

A triumph with necessary questions

The opening of the tunnel brought forth joy but with a tinge of doubt. The BBC News and The Guardian reported on the inauguration, noting excitement as well as concerns that the opening date came before safety and technical issues were fully addressed and that the safety assessment process lacked transparency.

According to the OECD’s “Strategic Transport Infrastructure Needs to 2030,” the Marmaray line has not only cut journey times but also opened up opportunities for work, reliability, safety, and bridged geographic and congestion divides while supporting the city's long-term development. The Council of Europe Development Bank (2026) argues that access to better mobility can lead to economic opportunity and social inclusion by increasing people's access to education, employment, and public services.

There are, however, unanticipated effects of connectivity. An OECD report titled “Transport Bridging Divides” notes that gentrification and displacement of longtime residents often see a rise and can be associated with the increase in property value around stations with transit improvements. Increased mobility within a city does not necessarily lead to more equitable access to the city. The paradox of transformative infrastructure is that each shortcut can also lead to a new destination, and a new inequality.

A tunnel beneath the water, a bridge between worlds

But perhaps Marmaray's most important feat isn't being the first to run trains under the sea, but how Istanbul functions as a metropolitan region. It is subtly changing what people move across borders. In Istanbul, the crossing offers a more everyday story of connection.

Here, Europe and Asia are linked not by symbolism alone, but by an ordinary commuter train. Every journey beneath the Bosphorus is a reminder that some of humanity's most enduring achievements are not monuments that separate us, but infrastructure that makes distance and sometimes division feel just a little smaller.