The Pantheon, Rome

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A domed rotunda in the Campus Martius, rebuilt in its present form under Hadrian and traditionally dated 126 CE, though brick stamps in the fabric point to construction chiefly between about 113 and 125. It still carries the largest unreinforced concrete dome in the world. The interior is a sphere resolved into a cylinder: the dome's internal span and the height from floor to oculus are both about 43.3 m, so a sphere of that diameter would touch the floor exactly. Almost every feature is a response to the material's lack of tensile strength - a drum wall around 6 m thick honeycombed with relieving niches and arches, aggregate graded from heavy travertine and tuff at the base to light pumice and scoria near the crown, five rings of coffers cutting dead weight out of the shell without weakening the compression paths, stepped rings of masonry buttressing the haunches from outside, and an open oculus about 8.8 m across at the crown where a keystone would carry no load anyway. Radial cracks in the shell were recorded by later surveys, and the building stands with them.