The Dome of the Roman Pantheon
The concrete dome of the Pantheon has been the world's largest self-supported dome for two thousand years. It is 142 feet high and made of concrete. It would flunk most city codes of our modern era due to the absence of steel rebar. A series of intersecting Roman arches supports the massive weight of the dome. The building has constantly been in use since that time - with a large hole in the ceiling! There are visible signs of wear on the stone floors from rain and sun... This image was taken at 17mm.