Istanbul is the only major city in the world that spans two continents: Europe and Asia. The city is divided by the Bosphorus Strait, which connects the Black Sea to the Sea of Marmara. Although Ankara is the official capital of Turkey, Istanbul remains the country's cultural, economic, and historical heart. For over 1,500 years, it served as the capital of three major empires: the Roman, Byzantine, and Ottoman. This diverse history is reflected in its architecture, most notably the Hagia Sophia, which has transitioned from a Christian cathedral to an Ottoman mosque and now a museum and mosque again. The city's geography has made it a strategic gateway for trade between the East and West for millennia. Today, residents can travel between continents in minutes via the Marmaray undersea rail tunnel or by the iconic ferries that crisscross the strait. Istanbul's Grand Bazaar remains one of the largest and oldest covered markets in the world, with over 60 streets and 4,000 shops.