The Bosphorus is the 32 km (20-mi)-long strait which joins the Sea of Marmara with Black Sea. It separates the two continents, Europe and Asia. The width of the Bosphorus varies from 500 meters (1640 feet) to 3 km (2 miles), its depth from 50 to 120 meters (164 to 394 feet), averaging about 60 meters (197 feet) deep.
If you are planing a self-guided tour make sure you have the ferry schedules. The first one starts at 10:35 from Eminonu.
The name "Bosphorus" comes from a Greek legend: Zeus had an affair with a beautiful women named Io. When Hera, his wife, discovered his infidelity, she turned Io into a cow and created a horsefly to sting her on the rump then Io jumped clear across the strait. Thus bous = cow, and poros = crossing-place: Bosphorus = "crossing-place of the cow."
Archeological research works carried out recently in the deep waters of the Black Sea have revealed the presence of sunken cities on the underwater slopes along the Turkish coast. The existing geological evidence supports the theory that in ancient times the northern end of the Bosphorus was blocked by earth and rock. The Black Sea had no outlet (like Lake Van today). And its water level was below that of the Aegean Sea, the Sea of Marmara.
However, an earthquake destroyed the Bosphorus blockage, releasing a deluge of water from the Bosphorus into the Black Sea, raising the water level and flooding its coastal communities. So it may well be that the Bosphorus is the source of Noah's flood and the legend of Noah's Ark! (Mount Ararat is also in Turkey).
The Bosphorus has been a waterway of the highest importance since ancient times. Ulysses passed through. Byzas, who founded Byzantium (later Constantinople, later Istanbul) sailed up and down looking for the perfect place to found his village.
In 1452, Mehmet the Conqueror ordered the construction of the mighty fortresses of Rumeli Hisari (Fortress of Europe) and Anadolu Hisari (Fortress of Anatolia) so he could control the strait and prevent reinforcements from reaching the besieged Byzantine capital of Constantinople.
To the Ottomans it was mostly an obstacle: each spring they had to ship their gigantic armies across the strait from Istanbul for campaigns in Anatolia, Syria and Persia.
During World War I, the Bosphorus was the key to the Black Sea and Russia. The Sultan held the key. The Entente powers wanted it. What they failed to get in by battle at Gallipoli campaign they got by treaty after the defeat of Germany World War I. The British finally had their warships anchored outside Dolmabahçe Palace.
