Optymistychna Cave

Optymistychna Cave (Ukrainian: Оптимістична, lit. 'optimistic') formerly known as Peshchera Optimistitscheskaya, 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. It is also the longest gypsum cave in the world.

Source: Wikipedia (CC BY-SA 4.0) — https://en.wikipedia.org/wiki/Optymistychna_Cave