Trebinje is the southernmost municipality and town in Bosnia and Herzegovina. It is administratively part of the Republika Srpska entity and is located in southeastern Herzegovina, some 10 km (6.2 mi) from the Adriatic Sea. The town was first mentioned in the 10th century. The town lies on the Trebišnjica River in southeastern Herzegovina, some 24 km (15 mi) by road from the city of Dubrovnik, Croatia on the Adriatic coast. There are several mills along the river, as well as several bridges, including two in Trebinje itself, and a historic Ottoman Arslanagic bridge nearby. The river is heavily exploited for hydro-energy. After it passes through the Popovo polje area southwest of the town, the river - which always floods in winter - naturally runs underground to the Adriatic, near Dubrovnik.