1. Optimistic CaveKorolivka Optymistychna is a gypsum cave located near the Ukrainian village of Korolivka, Borshchiv Raion, Ternopil Oblast. Approximately 230 km of passageways have been mapped within. As a result, it is the longest cave in Eurasia and the fifth-longest cave in the world, after Mammoth Cave, Sistema Sac Actun, Jewel Cave and Sistema Ox Bel Ha. It is also the longest gypsum cave in the world. From Wikipedia, the free encyclopedia.