Ottoman Sultan Abdulmejid is said to have proposed the idea of a tunnel under the Bosporus about a century and a half ago. One of his successors, Abdulhamid, had architects submit proposals in 1891, but the plans were not carried out.
The tunnel is just one of Erdogan's large-scale plans. They include a separate tunnel being built under the Bosporus for passenger cars, a third bridge over the strait, the world's biggest airport, and a massive canal that would bypass the Bosporus.
The projects have provoked charges that the government is plunging ahead with city-changing plans without sufficient public consultation. The concern fueled protests that swept Turkey in June.
Tuesday's ceremony on the 90th anniversary of the founding of the Turkish Republic was attended by Erdogan and other officials, including Japanese Prime Minister Shinzo Abe, whose country was heavily involved in the construction and financing of the railway tunnel project.
Japan's Seikan tunnel linking the Japanese islands of Honshu and Hokkaido is the world's deepest, getting 140 meters (460 feet) below the seabed and 240 meters (790 feet) below sea level. The Channel Tunnel linking Britain and France is as much as 75 meters (250 feet) below sea level.
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