A 2.5 hour bus ride into the mountains outside of Sofia in Bulgaria, Rila monastery dates back to the 10th century. It is credited as having helped save the Bulgarian language and spirituality during the 500 years of Ottoman rule. The church has the most detailed and impressive frescoes I’ve ever seen, though they date back to the 1850s (geez, practically brand new). In these mountains St. John of Rila founded Bulgarian monasticism (he went into the mountains to be a hermit and do his religious thing), and nowadays is considered the patron saint of Bulgarians. Google just told me there is even a Bulgarian Orthodox church in Chicago named after him.