Postojna Cave Park
The Cathedral room hosts two or three concerts per year, usually choirs. The entire cave system is six miles long with three levels; we visited the middle level. A train takes you more than a mile into the earth; then you walk almost another mile.
The Prince and Princess of Japan recently visited which may be why there were quite a few Japanese visitors, mostly women, mostly atwitter with giddy excitement.
The entrance to the caves looks like an amusement park but the second you get in the scale and smell and history of the place overwhelm the senses (in a good way.)