The Great Istanbul Tunnel, a 6.5km multi-use underwater tunnel traversing the Bosphorus river and connecting Europe and Asia, will serve 1.3 million passengers a day when it opens in 2028.

https://www.newcivilengineer.com/latest/worlds-longest-underwater-tunnel-under-construction-in-instanbul-and-scheduled-to-open-in-2028-12-05-2023/