Great Istanbul Tunnel

The Great Istanbul Tunnel (Büyük İstanbul Tüneli) is a planned multi-use highway and railway undersea tunnel in Istanbul, Turkey to cross the Bosphorus strait connecting the Europe side and Asia side. It will be the third and the longest tunnel crossing the Bosphorus strait, at a length of 6.5 km, surpassing the Eurasia Tunnel at 5.4 km and the Marmaray Tunnel at 13.5 km.

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Tunnel data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).

Other tunnels in Turkey

Delaware Aqueduct United States · 137 km Päijänne Water Tunnel Finland · 120 km Orange–Fish River Tunnel South Africa · 82.8 km Ishikawa Tunnel Japan · 78.5 km Brenner Base Tunnel Austria · 64 km Mont d'Ambin base tunnel France · 57.5 km Gotthard Base Tunnel Switzerland · 57.09 km Hong Kong–Zhuhai–Macau Bridge China · 55 km