Kocaeli Province (Turkish: Kocaeli ili, pronounced [kodʒaˈeli]) is a province of Turkey. Its capital is İzmit, which is also sometimes called Kocaeli. The province is the successor of the Ottoman-era Sanjak of Kocaeli. The largest towns in the province are İzmit and Gebze. The traffic code is 41. The province is located at the easternmost end of the Marmara Sea around the Gulf of İzmit. Kocaeli is bordered by the province of Sakarya and the Marmara Sea to the west, the Black Sea to the north, the province of Istanbul the east, the province of Bursa to the south and the province of Yalova to the southwest. The metropolitan area of Istanbul extends to the Kocaeli-Istanbul provincial border. The size and natural conditions of the Bay of İzmit allow for extensive port facilities, including the Gölcük Naval Base. The province is called the industrial capital of Turkey. Kocaeli has an airport named Cengiz Topel Naval Air Station which is used for military and civilian transport.