The world's deepest undersea tunnel connects Europe and Asia by rail
In short: For centuries, the Bosphorus strait served as a formidable barrier between continents. Now, a 13.6-kilometre underwater tunnel allows trains to glide beneath the waves. This engineering marvel isn't just a local transit link; it is a critical node in a modern "Iron Silk Road" connecting China to Europe.
The Marmaray project, which began in 2004, represents a massive logistical undertaking to link the European and Asian sides of Istanbul. It features a 77-kilometre commuter rail line running along the north shore of the Sea of Marmara. A defining feature is its 1.4-kilometre immersed tube tunnel under the Bosphorus strait. This structure was assembled from 11 sections, with some weighing up to 18,000 tons, and placed 60 metres below sea level—making it the world's deepest undersea immersed tube tunnel.
The project faced significant hurdles, most notably archaeological discoveries in 2005. Excavations at Yenikapı revealed an 8,000-year-old harbour, the city wall of Constantine the Great, and even a unique ancient galley. These finds delayed the first phase by four years as archaeologists documented evidence of settlement dating back to 6,000 BCE. Additionally, engineers had to account for extreme seismic risks. The tunnel is situated only about 18 kilometres from the North Anatolian Fault; to counter potential liquefaction in the silty soil, workers injected industrial grout 24 metres below the seabed and designed the walls to flex during tremors.
Financially, the project saw its costs nearly double from an original estimate of $4.5 billion. The Japan International Cooperation Agency (JICA) alongside the European Investment Bank (EIB) provided most of the funding. While carrying roughly 75,000 commuters each hour in one direction, cargo transit happens from 1:00 a.m. until 5:00 a.m., reducing the time for Chinese to Turkish exports by almost three weeks.
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- Summary by Intellect · source: Marmaray
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