Istanbul is located in northwestern Turkey within the Marmara Region on a total area of 5,343 square kilometers (2,063 sq mi). The Bosphorus, which connects the Sea of Marmara to the Black Sea, divides the city into a European side, comprising the historic and economic centers, and an Asian, Anatolian side; as such, Istanbul is one of the two bi-continental cities in Turkey, along with Çanakkale. The city is further divided by the Golden Horn, a natural harbor bounding the peninsula where the former Byzantium and Constantinople were founded. In the late-19th century, a wharf was constructed in Galata at the mouth of the Golden Horn, replacing a sandy beach that once formed part of the inlet''''s coastline. The confluence of the Sea of Marmara, the Bosphorus, and the Golden Horn at the heart of present-day Istanbul has deterred attacking forces for thousands of years and still remains a prominent feature of the city''''s landscape.
The historic peninsula is said to be built on seven hills, each topped by an imperial mosque, surrounded by 22 kilometers (14 mi) of city walls; the largest of these hills is the site of Topkapı Palace on the Sarayburnu. Rising from the opposite side of the Golden Horn is another, conical hill, where the modern Beyoğlu district is situated. Because of the topography, buildings were once constructed with the help of terraced retaining walls (some of which are still visible in older parts of the city), and roads in Beyoğlu were laid out in the form of steps. Üsküdar on the Asian side exhibits similarly hilly characteristics, with the terrain gradually extending down to the Bosphorus coast, but the landscape in Şemsipaşa and Ayazma is more abrupt, akin to a promontory. The highest point in Istanbul is Çamlıca Hill (also on the Asian side), with an altitude of 288 meters (945 ft).
Istanbul is situated near the North Anatolian Fault on the boundary between the African and Eurasian plates. This fault zone, which runs from northern Anatolia to the Sea of Marmara, has been responsible for several deadly earthquakes throughout the city''''s history. Among the most devastating of these seismic events was the 1509 earthquake, which caused a tsunami that broke over the walls of the city, destroyed over 100 mosques, and killed more than 10,000 people. More recently, in 1999, an earthquake with its epicenter in nearby İzmit left 17,000 people dead, including 1,000 people in Istanbul''''s suburbs. The people of Istanbul remain concerned that an even more catastrophic seismic event may be in Istanbul''''s near future, as thousands of structures recently built to accommodate the city''''s rapidly increasing population may not have been constructed properly. Seismologists say the risk of a 7.6-magnitude earthquake striking Istanbul by 2030 is greater than sixty percent.