Two underground rivers dissolved Baradla out of the limestone and left it. The guides call them the Styx and the Acheron, and they run well below the walkway now, coming back up into the main passage only on a few flood days a year, after snowmelt or hard spring rain. What they cut is the longest cave in Hungary: 6.6 kilometres of dripstone corridor under the Aggtelek karst, carrying on under the Slovak border. You go in where the hillside simply opens at the foot of a high white cliff, and the temperature drops at once to ten degrees. Underfoot is the old streambed, dry and gravelly. Most of the route is narrow, and the formations stand close: the Christmas Tree under a single lamp, a tiered cone of pale calcite that the water is still building drip by drip. Then the Giants' Hall, where the walls pull back so far that the lamps stop reaching the far side, and a stairway switchbacks down through boulders and dripstone to the floor.
Partway along, the group files into a chamber laid out with seats. The lights drop and music is played into the dark. Bare limestone reflects sound instead of soaking it up, so every note hangs in the air for seconds after the player has stopped. Choirs and orchestras have been performing down here for two centuries. The long route takes you along the whole main branch, hours of it, until a guide unlocks a timber door and the beech forest is suddenly there again: you went in at one village and you come out at another. Twenty-one species of bat sleep out the winter in the side branches, in air that never freezes. One chamber on the way is called the Bone House, and it earned the name. Thirteen people were buried there face down around 1200 BC, each with a flat stone laid across the back.