Professional Roman surveyors located the inlaid marble floor to conform to a convex contour, which drained away the rain from the oculus for hundreds of years. The section of the Pantheon in Figure 4., shows the distribution of the different types of caementa used: from the heaviest, travertine, at the bottom to the lightest, volcanic scoria and yellow tuff, at the top. It is noteworthy that the whole dome is not made as light as possible; that only the crown has the lightweight scoria and yellow tuff. [3]