Değirmendere Merkez, Gölcük

Gölcük Naval Base (Turkish: Gölcük Deniz Ana Üssü) is the main base of the Turkish Navy on the east coast of the Sea of Marmara in Gölcük, Kocaeli. It is the principal base for logistic support with various facilities stretched over 1,800 acres (7.3 km2) of land. In addition to the workplaces, the base houses apartment-like barracks, social facilities, a military hospital, a military museum and an archive for naval history. The shipyard at the base is capable of constructing frigates, submarines, fast attack boats and auxiliary vessels with high technology in recognized standards.

Gölcük Naval Shipyard (Turkish: Gölcük Donanma Tersanesi) is a naval shipyard of the Turkish Navy within the Gölcük Naval Base on the east coast of the Sea of Marmara in Gölcük, Kocaeli. Established in 1926, the shipyard serves for the building and the maintenance of military vessels. A total of 3,221 personnel are employed at the shipyard stretching over an area of 255,526 m2 (2,750,460 sq ft) with covered structures of 121,466 m2 (1,307,450 sq ft).

İzmit Körfez Circuit (Turkish: İzmit Körfez Yarış Pisti) is a motor sports race track about 17 km (11 mi) west of İzmit, in Körfez district of Kocaeli Province, Turkey. Sponsored by the Turkish Automobile Sports Federation (TOSFED) and built by the Istanbul Motorsports Club (İMK), it was inaugurated in 1993 as the first asphalted race track in Turkey. The venue is home to motor sport races in various categories. Located at the northeastern coast of Marmara Sea, İzmit Körfez Circuit is situated around a small Fener Lake (Fenergölü), which belongs to the Municipality of Körfez District. The venue can be easily reached from Istanbul and İzmit by the motorway O-4 or highway D-100 at the exit Körfez. The race track, 1.950 km (1.212 mi) at length, has nine turns and runs clockwise. The circuit provides the greatest number of motor sport racing events in Turkey. In addition to Turkish championships, several racing organizations of private companies as well as various motor sport types in different categories are held here. On an average racing day, several events are carried out in five to six categories. The circuit is used also for test runs by motor magazines, tire and motor companies. Some motorcycle and auto racing schools train here also. Around 5,000 spectators in every age group come to the circuit on racing days. Events at the race track find growing interest in the TV and press.

Elaea or Elaia (Ancient Greek: Ελαία) was a town of ancient Bithynia on the coast of the Sinus Astacenus in the Propontis. Its site is located near Zeytin Burnu (Körfez), in Asiatic Turkey.

Tüpraş Izmit Oil Refinery (Turkish: Tüpraş İzmit Rafinerisi) is an oil refinery in Izmit, western Turkey. It is owned and operated by Tüpraş. Amongst others it refines Urals crude. Its capacity is slightly less than that of Tüpraş Izmir. It produces diesel fuel. Climate Trace estimates that in 2023 it emitted almost 3 million tonnes of greenhouse gas.

Gölcük, formerly known as Diolkides, is a municipality and district of Kocaeli Province, Turkey. Its area is 217 km2, and its population is 175,940 (2022). The city is located at the northern gulf of Armutlu Peninsula on the coast of Gulf of İzmit, a branch of the Sea of Marmara, in the south of the province. It is the district, where the 1999 earthquake disaster happened. Gölcük is the location of one of the Turkish Navy's main naval bases, and of a Ford Otosan automobile plant. The mayor is Ali Yıldırım Sezer (AKP).

Gölcük Barbaros Hayrettin High School (Turkish: Gölcük Barbaros Hayrettin Lisesi), is a public high school in Gölcük, Kocaeli, Turkey. The school is named after Ottoman admiral Hayreddin Barbarossa. Short form of the school name is GBHL. Formed in 1958 as "Gölcük Lisesi" (GL), the high school named as "Barbaros Hayrettin Lisesi" in 1970 when it moved to its own building from the provisional location. The colors of the school are black (symbolizing the deadly ferry accident on 1 March 1958 in which hundreds of high school students died in Gulf of İzmit) and yellow (symbolizing the sun/GBHL rising after this disaster). In the devastating earthquake epicentered in Gölcük on 17 August 1999, GBHL lost many of its students and faculty residing in Gölcük and surrounding towns. The buildings of the high school has been damaged, however later been repaired by the efforts of its alumni association and some international relief organizations.

Derince is a seaport, municipality and district of Kocaeli Province, Turkey. Its area is 198 km2, and its population is 146,374 (2022). The mayor is Zeki Aygün (AK Parti). Derince Port is one of the few ports in Turkey under government control. The port is operated by the Turkish State Railways (TCDD). Having one of the biggest storage area in the region, the port is widely used for finished vehicle product transportation. Derince is one of the few ports with railway connection in that area, and has the longest rail network within the port. Two rail ferry services use Derince Port, Tekirdağ-Derince operated by TCDD and Derince-Chornomorsk/Poti operated by Bati Wagon. The port is in the privatization list of Privatization Administration. The privatization process had restarted in October 2013. Six companies had offered for Derince Port, but the tender process was cancelled when none of them continued in the public auction.

USS PC-1640 was an PC-1638-class submarine chaser in the United States Navy during the Cold War. She was transferred to the Turkish Navy as TCG Yarhisar (P 113) of the Hisar-class patrol boat.

Derince railway station (Turkish: Derince istasyonu) is a railway station in Hereke, Turkey. TCDD Taşımacılık operates four daily regional trains between Istanbul and Adapazarı that stop at the station. The station was originally built in 1904 by the Ottoman Anatolian Railway along with the Port of Derince.Derince station was closed down on 1 February 2012 due to construction of the Ankara-Istanbul high-speed railway. The station was reopened on 4 August 2017.

Ford Otomotiv Sanayi A.Ş. (Turkish for 'Ford Automotive Industry'), doing business as Ford Otosan, is an automotive manufacturing company based in Turkey that is equally owned by Ford Motor Company and Koç Holding. The company was established in its current form in 1977, with original relations dating back to 1928. It currently operates in six locations: Gölcük and Yeniköy plants in Kocaeli, İnönü plant in Eskişehir, Craiova plant in Romania, Sancaktepe R&D Center and spare parts warehouse in Istanbul The company employs more than 20,000 people and had a production capacity of over 700,000 vehicles, 400,000 engines, and 140,000 powertrains by 2022.

Diolkides was a town of ancient Bithynia. Its site is located opposite Zeytin Burnu, Gölcük, in Asiatic Turkey.