Istanbul is the largest city in Turkey, constituting the country's economic, cultural, and historical heart. With a population over 15 million, it is home to 18% of the population of Turkey. Istanbul is among the largest cities in Europe and in the world by population. It is a city on two continents; about two-thirds of its population live in Europe and the rest in Asia. Istanbul straddles the Bosphorus—one of the world's busiest waterways—in northwestern Turkey, between the Sea of Marmara and the Black Sea. Its area of 5,461 square kilometers (2,109 sq mi) is coterminous with Istanbul Province.