East of Mostar is the spring of the Buna River, and it is the largest karst spring in Europe with an average annual flow of 43 m³ per second. The spring is located below a 200m high stone cliff, and the Dervish Tekija was built nearby, which was mentioned in 1664 by the travel writer Evlija Ćelebija. The water springs from a 17-meter-wide cave, and according to research, the underground flow of the Buna spring is about 200 m long. Throughout the year, the water is extremely cold and clean, which favors the development and habitat of various endemic species, such as brown trout. The river is 9 km long and finally flows into the Neretva.