In today’s section of Dom Guéranger’s “The Liturgical Year” discussing the feast of Mary, Mother of God, he mentions:
The holy Pope Xystus 3rd ordered an immense Mosaic to be worked into the Chancel-Arch of the Church of St. Mary Major, in Rome, as a monument to the holy Mother of God.
I was hoping to find a picture, but the Vatican has done even better than that in creating one of those panoramas where you can look around the church. Here is a link to it (it worked on my Chrome browser, but not my Firefox browser). Here is a screenshot of it:
If you’re interested in Guéranger’s Liturgical Year, you can download it for free as pdfs. I got my originals from here. The best way I’ve found to read it on a somewhat regular basis is by using the Kindle App on a phone or tablet (Kindle e-ink readers don’t do a good job with pdfs). To better manage them in the Kindle app, I edited some of the metadata and have it shared in a public google drive folder here. Here are some screenshots from my iOS device showing 2 of the volumes in my bookshelf, how easy it is to quickly flip through hundreds of pages, and that it’s actually readable on a phone.