The İstanbul Strait links the Black Sea, referred to in ancient times as Pontius Euxinus, with the Sea of Marmara, known as Propontis. The İstanbul Strait was formerly known as Bosporos and there is a legend to enrich this name.
It is considered that the root of this name, “Bos”, is of Thracian origin. The length of the Strait ranges between 28.5km and 31.7km (depending on where it is measured), which means an average length of 30km. Its width at the northern end is 4700m, and at the southern end, 2500m. The widest place in the Strait is in the Büyükdere vicinity (3300m), and the narrowest place is that which lies between Rumelihisarı and Kanlıca (660-700m). It is known that the deepest part of the Strait is 100m in depth. Apart from this, there are two main currents in the Strait; one of these is the surface current, flowing from the Black Sea to Marmara, this could be considered as the surplus waters of the Black Sea. The other current flow is along the bed of this channel from Marmara towards the Black Sea under the hydrostatic pressure of salty, heavy density water. A number of hypotheses exist about the formation of the İstanbul Strait;
1. That the İstanbul Strait came about as a result of the rocks being worn away by friction; that the surplus waters of the Black Sea, which is an inland sea, gradually wore away the channel now known as the İstanbul Strait.
2. That the İstanbul Strait came into being as a result of volcanic action and that the water of the Black Sea flowed through the fissure that opened up.
3. That the İstanbul Strait was originally a valley which gradually filled with water.