The Dardanelles is the narrow strait that lies between the Aegean Sea and the Sea of Marmara. Its northern shore is formed by the Gallipoli Peninsula, part of the European land mass, and its southern by Turkey's Asiatic coast. The strait is 61 kilometres long and a strong surface current flows in the direction of the Aegean, which is echoed by an undercurrent flowing toward the Sea of Marmara. The Dardanelles have always been of great strategic importance because they link the Black Sea with the Mediterranean Sea and provide the only seaward access to the ancient city of Constantinople (Istanbul). During the First World War, Turkey heavily fortified the Dardanelles with both minefields and shore batteries. British and French vessels made an attempt to force their way through these defences on March 1915 but were thwarted by the Turkish guns and mines. The Gallipoli landings were subsequently mounted in an attempt take control of these shore batteries.