- Optymistychna Cave
- Ukraine ()
Optymistychna Cave, near Korolivka, Ukraine, is a cave system that occurs in a layer of gypsum around 20 m in thickness, and which has been mapped so far at 236 km (146 miles) in length. The cave network lies under an area of around 2 km². It was discovered in 1966 and is the largest cave network in Eurasia.