This photo, taken from the belltower, shows the dome on the Cathedral in Florence. Begun in 1420 by Brunelleschi, this dome is the largest church dome in the world and served as the model for the dome on St. Peter's. An engineering marvel, it is actually two domes -- the exterior dome seen in this photo and an interior one. The two are connected by a common ribbing, giving the required structural stability. Tourists climbing the stairs to the "lantern" area at the top of the dome actually climb between the two domes.