Optymistychna Cave, Gypsum karst cave in Ternopil Oblast, Ukraine.

Optymistychna Cave is a gypsum karst system in Ternopil Oblast featuring over 264 kilometers of mapped passages, with tunnels typically measuring about 10 feet (3 meters) wide and 5 feet (1.5 meters) tall. The network extends through multiple levels with varying corridors, from cramped channels to larger chambers carved over millennia by water erosion.

Speleologists from the Lviv club Cyclope discovered this underground network in 1966, launching systematic exploration of the site. More than 50 expeditions followed, gradually mapping the passages and establishing it as one of Europe's longest cave systems.

The name comes from the Ukrainian word for optimistic, given because sceptical colleagues called the Lviv speleologists who found the cave "optimists". The cave lies in the Korolivka forest tract and is managed by the Lviv speleological club Cyclope, which received the nature monument status in 1971.

The cave is not flooded, but seasonal floods can submerge the entrance area and block access for several days to two weeks. The entrance lies in the Korolivka forest tract southwest of the village, about 115 km from Ternopil. Exploration is managed by the Lviv speleological club Cyclope, and there are no guided tours for visitors.

Helictites found here overturned the theory that such formations grow only in limestone caves. The cave also holds a small lake, Aqva vitae, with a surface area of 82 square metres, and more than 260 expeditions have mapped its passages.