The Great Istanbul Tunnel, a 6.5km multi-purpose underwater tunnel crossing the Bosphorus River and linking Europe and Asia, is set to cater to 1.3 million passengers daily when it opens its doors in 2028The Great Istanbul Tunnel, a 6.5km multi-purpose underwater tunnel crossing the Bosphorus River and linking Europe and Asia, is set to cater to 1.3 million passengers daily when it opens its doors in 2028