TURKEY has opened world's first underwater tunnel connecting two continents, fulfilling an Ottoman sultan's dream 150 years ago in a three-billion-euro mega project. The Marmaray tunnel runs under the Bosporus, the strait that connects the Black Sea to the Sea of Marmara and divides Istanbul between Asia and Europe. The tunnel is 13.6km long, including an underwater stretch of 1.4km. It is among a number of large infrastructure projects under the government of Turkish Prime Minister Recep Tayyip Erdogan that have helped boost the economy but also have provoked a backlash of public protest.