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Koljčin Skok is a remote waterfall on Stara Planina's Štavljenska river, near the village of Topli Do (~1,550 m altitude). It cascades roughly 17 m amid rugged cliffs and dense forest. Visitors reach it via a steep and adventurous hiking trail that also passes other smaller falls like Čunguljski and Kurtulski skok. It's one of the most challenging yet rewarding sights on the mountain — ideal for experienced hikers seeking wild, off-the-beaten-path nature.