Baradla Cave is the longest cave in Hungary, a corridor of dripstone running 6.6 kilometres under the Aggtelek karst in the northeast of the country and on across the Slovak border, walked from one village entrance to the next through halls big enough to seat an orchestra, and its largest chamber is a cathedral nave whose roof is beyond the reach of the lamps. The cave was cut by two underground streams that the guides named the Styx and the Acheron, and it has been visited since the eighteenth century, a UNESCO site; the entrance at Aggtelek is at the foot of a high white limestone wall where the hillside simply opens, and the temperature drops at once to ten degrees, with the short tour running an hour through the first halls and the long tour crossing the hill underground for seven kilometres to come out in the woods at Jósvafő. The Giants' Hall opens overhead with a stairway switchbacking down through boulders and dripstone to the floor, and the Christmas Tree stalagmite, a tiered cone of pale calcite under a single lamp, is the formation everyone stops to photograph.
Partway along the route the group files into the Concert Hall, a chamber laid out with seats, the lights drop and music is played into the darkness, the acoustics of the stone being the reason concerts have been held in the cave for two centuries. One chamber is called the Bone House, and it earned the name, since thirteen people were buried there face down with a flat stone laid across the back, from the Bronze Age, and the cave held people from the Neolithic on, whose pottery gave a culture its name. After snowmelt and heavy spring rain the streams rise back into the main passage and run along the streambed the tour walks, a handful of days a year and never on schedule, when the cave is closed. Baradla is a road under a hill, and the villages at either end are its doors.