Turkish President Recep Tayyip Erdogan opened the first road tunnel linking Europe and Asia.
The Eurasia Tunnel was built under the Bosphorus seabed and is the latest project in Turkey's plan to alleviate Istanbul's notorious traffic congestion.
Construction of the tunnel took over five years. A 5.4 kilometre (3.3 mile) section of the 14.5 kilometre (9 mile) route was built beneath the Bosphorus strait.
The tunnel cost $1.3 billion. It is expected to reduce travel time between the European and Asian sides of Istanbul to just 15 minutes.
A trip through the tunnel will initially cost 15 lira ($4.25) with revenue going to families of those who died in recent terrorists attacks.
TRT World's Jon Brain has more.