This Is What A 1,900 Year Old Concrete Dome Looks Like

The ceiling of the Pantheon in Rome.  Built during the reign of Emperor Hadrian (117-138 AD), after nearly 2,000 years it remains the largest unreinforced concrete dome in the world.  The best preserved of the buildings of ancient Rome due to its conversion into a church in 609, the external facade has suffered damage including stripping of bronze roof tiles by the Byzantine Emperor Constans II in 663, and removal of marble cladding at various points over the centuries.  The photo is remarkable, seeing it in person even more so.