Once inside the Pantheon, turn around and look just above the main doors. There is a small rectangular hole in the interior surface of the dome. Brunelleschi cut the hole to investigate its construction when he was planning the Duomo in Florence (which took over the title of largest dome in the world from the Pantheon, after the latter had held it for 1300 years). Next, read “Brunelleschi’s Dome”, a short but fascinating book about the Duomo, and Brunelleschi’s competition with Ghiberti.