The Optymistychna is a gypsum cave located near the Ukrainian village of Korolivka, Chortkiv Raion, Ternopil Oblast. Approximately 264 km (164 mi) of passageways have been mapped within. As a result, it is the longest cave in Europe and the seventh-longest cave in the world, after Mammoth Cave, Sistema Sac Actun, Jewel Cave, Shuanghedong and Sistema Ox Bel Ha. It is also the longest gypsum cave in the world.