Filled with karst springs and enormous, spectacularly lit chambers, the Grotte de la Luire was used as an emergency hospital during WWII. When it's open, up to five tours depart daily (a dozen daily in July and August), but days of operation vary greatly throughout the year: check the website to be certain.
Standard visits arm you with a lamp, but for added atmosphere, it's worth reserving a spot on the occasional torchlit night tours.