Gölcük, historically known as Diolkides, is a municipality and district of Kocaeli Province, Turkey. Its area is 217 km, and its population is 175,940 (2022). Gölcük is located on the southern shore of the Gulf of İzmit, which is the easternmost inlet of the Sea of Marmara.
Gölcük is home to the Gölcük Naval Base and Gölcük Naval Shipyard of the Turkish Navy. A Ford Otosan automobile plant is also located in the district.
Gulf of İzmit (Turkish: İzmit körfezi), also referred to as İzmit Bay, is a bay at the easternmost edge of the Sea of Marmara, in Kocaeli Province, Turkey. The gulf takes its name from the city of İzmit. Other cities and towns around the bay are Gebze, Körfez, Gölcük, and Altınova.
In the east–west direction, it extends for a length of about 48 kilometres (30 mi), while in the north–south direction its width varies from 2 to 3 kilometres (1.2 to 1.9 mi) at the narrowest spots to about 10 kilometres (6.2 mi) at its widest. The İzmit Bay Bridge is a suspension bridge that bridges the gulf.
İzmit (Turkish pronunciation: [ˈizmit]) is a municipality and the capital district of Kocaeli Province, Turkey. Its area is 480 km, and its population is 376,056 (2022). The capital of Kocaeli Province, it is located at the Gulf of İzmit in the Sea of Marmara, about 100 km (62 mi) east of Istanbul, on the northwestern part of Anatolia. Kocaeli Province (including rural areas) had a population of 2,079,072 inhabitants in 2022, of whom approximately 1.2 million lived in the largely urban İzmit City metro area made up of Kartepe, Başiskele, Körfez, Gölcük, Derince and Sapanca (in Sakarya Province). Similar to Istanbul, the area of İzmit is coterminous with its province. It is also the most populated of any city or town in Turkey whose name isn't shared with the province it is located in.
İzmit was known as Nicomedia and Ólbia in antiquity, and was the eastern and most senior capital city of the Roman Empire between 286 and 324, during the Tetrarchy introduced by Diocletian. Following Constantine the Great's victory over co-emperor Licinius at the Battle of Chrysopolis in 324, Nicomedia served as an interim capital city for Constantine between 324 and 330. During the Ottoman Empire, İzmit was the capital of the Sanjak of Kocaeli. In the present day, Istanbul-İzmit area is one of the main industrial regions in Turkey.