The population increase in Istanbul is also reflected in car ownership, which brings an extensive traffic load to the city, especially to the two bridges over the Bosphorus. After feasibility studies, a double deck, 3.4 km subsea tunnel with a 13.7 m diameter at a maximum depth of 106 m below sea level and a capacity of 100,000 cars/day was initiated as a solution for the city. The tunnel, excavated by a custom-made mixshield slurry TBM, passes through very complex geological structures including the Trakya formation, transition zones and marine sediments in a highly seismic area.