Oh, did I mention that it's friggin' huge? Of course, it's in Rome, so that's to be suspected.

One sort of strange element of the cathedral is the paintings. At first glance you look at the walls and they appear to have huge canvases, intricate carvings, be filled with alcoves containing cherub statues, covered in flowing tapestries, or supported by thick marble pillars. But, upon further inspection (and when things are far away, sometimes you can't even be sure) you notice that some of it is actually just painted onto the walls with incredible realism. It's just like we do in 3d graphics: wherever the people can get close, you actually spend the detail to make it nice. But far away, you just paint it in a clever way to make it look more detailed than it really is. Texture-mapping has a much longer history than I realized.