Slovakia is well known for its underground caves, and one of the largest is Domica. The cave is located in the Slovak Karst National Park and is connected with Baradla cave, which is found in Hungary. It's home to the underground Styx River and almost 1,500 bats. Domica cave will definitely amaze you with its different sinter fills, stalagmites, massive domes, Roman baths, and many other exciting things.
The cave is available for exploring from February until December, including boating. However, the best boating experience is expected in summer months between June and August when river Styx is full of water. Summertime is the high season, so be ready that it can be pretty crowded and challenging inside. Even in warm season, the temperature in Domica cave is approximately 11 degrees Celsius, so don't forget to take a sweater.