Izvor Kupe or Kupa Source, an area of 10 hectares, was declared a protected natural habitat on 12th December 1963. The Kupa Source is one of many still unresolved karst mysteries. It is also one of the most water abundant and deepest Croatian springs. At an altitude of 321 m, the water emerging from an underground spring has formed a beautiful turquoise lake. The research found this spring uses two or more vertical type of canals in the shape of a vertical well to reach the top. In the summer of 2008 more scientific research was successfully achieved by one of the worlds most famous Italian diver, Luigia Casatia. On 7th August 2008, Luigia dived at the incredible depth of 154 m and did not reach the bottom. Some 30 years ago the tests have been done using special watercolour to establish the connection and the path of Kupa Spring Source. The tests showed an underground connection with Malenica spring and Velika Voda spring near Crni Lug as they both emerge at the Kupa Spring Source. The average water outflow of the source is approximately 1200 l / s, whilst maximum water quantities are not measurable. Due to the richness of flora and fauna, including an abundant variety of butterflies, the upper course of Kupa has been named the Magical Butterfly Valley. One can reach the Kupe source from two directions; via the village of Razloge which is 15 km away from Crni Lug, and/or via Kupari village,30 km away. Due to narrow roads, we recommend the entry via Razloge by car. From Razloge follow the hiking trail for 30 minutes to reach the source. The hike journey back is longer. You can also reach the source from the village of Hrvatsko (Crni Lug - Hrvatsko 45 min drive) via Kupari. (Hrvatsko - Kupari 1h 30min walk) The path from Kupari runs along the river and after 30 minutes it will bring you to the source. On your way back you can use the path that runs right along the river, parallel to the river.