You cannot miss these caves as you drive from Manacor to Porto Cristo. Most people only want to visit one set of caves during their time on Mallorca and the giant billboards and flags at the entrance are an attempt to ensure that this is the one. In fact you are better off continuing to the 'Cuevas del Drach' or up the coast to the 'Coves d'Arta'.
But for serious speleologists, here are the facts. The caves were discovered by Pedro Caldentey in 1905 and the electric lighting was added by his son. Their name means 'fish hooks', which the stalactites are said to resemble. You get a guided tour and, yes, another concert on an underground lake.
The caves also make for a wonderful venue for weddings and parties.