Istanbul is long known as a city endowed with layer upon layer of cultural riches dating from ancient times. But beyond the obvious sites in the Old City, there is a jewel that’s rarely seen: the Kariye Museum. It’s not easy to do but it is well worth the effort. The interior walls are covered with exquisite Byzantine religious art that has no equal anywhere in the world. Despite earthquake damage in 1894, many spectacular mosaics remain, awesome in their fine detail, displaying subtle shadings and rich colors that are lacking in earlier Byzantine mosaics.