I'm making researches on a few underground places of worship in Italy, so it would be of a great help if I could compare with similar places in Europe.
perhaps double check on the churches near Paris catacombs as well? Would also check texts on Lalibela in Ethiopia and all texts that deal with medieval Cappadocia - not in Europe though ...
Hi. In addition to the Medivial ones in Cappadocia as Jelena Bogdanovic suggested you may want to check also those were in Antakya and Tur Abdin region in Turkey.
La Cueva de la Camareta (Agramón, Hellín-Albacete), seems another spanish site that may be of interest (published in Antigüedad y Cristianismo, X (1997)
many temples of Mithras are underground. They represent a cave and sometimes are actually in caves. For good surviving underground examples, try the mithraeum of the circus Maximus in Rome, the mithraeum of Santa Prisca, the mithraeum of San Clemente, the mithraeum of San Stefano Rotondo, etc, all in Rome, one or two of the mithraea in Ostia, and others scattered throughout the Roman Empire. A good book – sorry, useful is the better adjective – is still Vermaseren's Corpus Inscriptionum et Monumentorum religionis Mithriacae. for Mithras in Ostia you might try www.ostia-antica.org.