If you are interested in Catholic history, the cave churches of Cappadocia in Turkey are quite interesting to learn about. The volcanic ash formed rock formations over many years which were then carved out and used as houses and churches. These formations contain hidden entrances and passages and during times of persecution they were used as shelters. Saint Paul even traveled to this area in his efforts to spread the gospel. The Christian images preserved in these cave churches are another testimony to Catholic Tradition. You can learn more about it here and here.