From narthex to sanctuary and up to the apex of the 143ft central dome, 41.5 million pieces of glass tesserae in 8,000 colors encrust 83,000sq ft. Byzantine and Italian in style, the spectacle represents the largest mosaic collection in the world. Construction began in 1907, and the mosaics took 20 artists over 75 years to complete. A museum about the church and its mosaics occupies the lower level.