Subterranean Spectacular

Visitors usually liken the Postojna caves in southwestern Slovenia to the interior of a cathedral — cavernous, soaring, sublime. The caves, first opened to the public in 1819, are a 23-km warren of underground galleries, chambers and tunnels, all adorned with stalactites and stalagmites formed over the past 2 million years. In the 1820s, one of the largest chambers was the venue for grand balls, complete with candlelight and festive decorations. In those days visitors rode into the caves in horse-drawn carriages.

With the installation of electric lighting a century ago, the largest and...