By 2028, millions of people will be able to travel between two continents via an underwater tunnel that is projected to cost around $4 billion (￡3.1 billion).

       The Great Istanbul Tunnel, a four-mile multi-purpose underwater tunnel, will cross the Bosphorus River - a natural strait located in Turkey’s largest city which connects the Black Sea to the Sea of Marmara and forms one of the continental boundaries between Europe and Asia.

       The tunnel will feature three levels: two for road traffic and one for a rail system.

       It will have a diameter of 19 metres, be situated 110 metres below sea level and will connect the highways between Istanbul’s three airports - Istanbul Atatürk Airport, Sabiha G？k？en Airport, Istanbul Airport - and the nine rail lines of the Istanbul Metro.

       The railway section of the Great Istanbul Tunnel will comprise 13 stations, accommodating 70,000 passengers an hour in one direction.

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       It will also connect with 11 other railways, and allow the Metrobus line, a crucial part of Istanbul's transportation system, to function at optimal capacity.

       Turkey’s transport and infrastructure minister Adil Karaismailo？lu unveiled an update on the plans for the project on May 10, confirming the timeline for the opening of the combined highway and railway tunnel and additional details on the project, expected as the world’s longest underwater tunnel.

       Karaismailo？lu said the scheme would offer rail travel times across the Bosphorus of just over 40 minutes.

       The project has been deemed vital for the area. Data from Istanbul’s transportation plan indicates that daily crossings between the European and Asian sides of the river are set to exceed three million in the future, a significant increase from the current figure of two million.

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       Plans are also underway for a high-capacity rail system running from west to east across Istanbul, the minister said.

       The Great Istanbul Tunnel project is being excavated using one of the world's largest tunnel machines (TBM), an impressive piece of machinery measuring 83 metres long and 20 metres wide.

       This marks the third tunnel crossing the Bosphorus, following the construction of the two-mile Marmaray Tunnel and the 3.3-mile Eurasia Tunnel.

       Ministers claim it will be the world’s longest underwater tunnel. However, the 11-mile Fehmarnbelt tunnel between Denmark and Germany is currently under construction, due to be completed in 2029. It will cross a stretch of the Baltic sea and have a dual road and rail system.