AggtelekHungary, close to the Slovak border.
Many caves are available:
Béke5-hour tour, good shoes needed, and helmets are used - only part of the cave is paved.
Vass Imre6.5-km/3-hour tour, 6 km of which is spent between ticket office and the cave. Temporarily closed.
Rákoci Cave400-m/40-min tour, claimed to be probably the most spectacular cave in the region.