Bosporus (bŏsˈpərəs) [key] or Bosphorus –pərəs [Gr., = ox ford, in reference to the story of Io], Turk. Boğaziçi, strait, c.20 mi (30 km) long and c.2,100 ft (640 m) wide at its narrowest, separating European from Asian Turkey and joining the Black Sea with the Sea of Marmara. İstanbul is on the Bosporus. At its narrowest point stand two famous castles: Anadolu Hisar (1390) on the Asian side and Rumeli Hisar (1452) on the European side. With the Dardanelles, the Bosporus connects the Black Sea with the Mediterranean; it is thought to have been a dry riverbed as recently as 7,600 years ago. The Bosporus Bridge, one of the world's longest suspension bridges (3,524 ft/1,074 m long; opened 1973) spans the strait at İstanbul. A second bridge was completed in 1988, and a railroad tunnel under the strait opened in 2013. In 2011 the Turkish government proposed building a canal parallel to the Bosporus to reduce the shipping congestion in the strait.