Trebišnjica River is underground river in Herzegovina, and one of the largest underground rivers in Europe. It is 96.5 km long and rises at an altitude of 398 m. The Trebisnjica River flows through Gatačko, Cerničko and Fatničko Polje, where it sinks into the earth, only to reappear south of Bileća under the name Trebišnjica. It also appears as spring of Dubrovnik’s river Ombla, and the most part of Trebišnjica flows underground in the Neretva River.