The 3-billion-euro ($4.1 billion) rail tunnel is 13.6 kilometers (8.5 miles) long, 1.4 kilometers of which lie under the Bosphorus, the strait dividing Istanbul between Asia and Europe.

The tunnel is part of a larger "Marmaray" rail-infrastructure project whose title combines the name of the Sea of Marmara, which lies just south of the site, with the Turkish word for rail, "ray."

It is designed to accommodate 1.5 million passengers per day, thus easing traffic problems in Turkey's largest city, particularly over the two bridges that currently connect the two sides of the city.

Two million people cross the Bosphorus each day on the bridges, often creating massive congestion.

Turkish officials say that the rail tunnel is more than 60 meters (nearly 200 feet) under water, making it the deepest of its kind in the world.

The tunnel is one of a large number of infrastructure projects undertaken by the government of Prime Minister Recep Tayyip Erdogan. While seen as boosting the economy, they have also aroused some public protest from people who say the urban development is forcing people from their homes and destroying green spaces.

Archaeological delays

Construction of the tunnel began in 2004 and was originally scheduled to take four years. However, a number of important archeological finds - including a cemetery of some 30 Byzantine ships - delayed its completion. Some 40,000 objects were salvaged from the site.

Ottoman Sultan Abdulmejid is said to have first proposed building a tunnel under the Bosphorus a century and a half ago. However, at the time, technology was not sufficiently advanced to carry out the project.

Erdogan revived the plan in 2004.

More projects planned

The prime minister is planning to carry out several more infrastructure projects in the future, including a separate tunnel under the Bosphorus for road vehicles, a third bridge over it, the world's biggest airport and a massive canal bypassing the waterway.

Tuesday's ceremony to inaugurate the tunnel coincides with the 90th anniversary of the founding of the Turkish Republic.

The occasion will be attended by Japanese Prime Minister Shinzo Abe, whose country has been heavily involved in the construction and financing of the project.

tj/slk (AP, AFP)