These gave rise to same jokes, like the one of the enormous rats eager of marble, or to the wrong conviction coliseum was damaged during II World War bombings.
Explanation has to be found in the dark ages following the fall of the Western Roman Empire in 476AD.
Roman population dramatically decreased as a result of the barbarian invasions, plagues, earthquakes and floods which affected the city from year 400 to 500 AD, as to slow down the inhabitants to few thousand people (estimate between 50.000 to 80.000 in 500AD, as opposite to 700.000/900.000 of 400AD).
Rome experienced a long period of extreme poverty and uncertainty; it remained the centre of world’s richness only in the mind of the Barbarians populating the north. The city became small and under populated: the remind of the great achievement of the Roman culture were lost and the citizen’s life was dominated by Christian religion and survival needs.
About the year 600AD an edict issued in Byzantium entitled the Romans to spoil ancient buildings which were not any longer in function.
Buildings became caves of building materials employed to erect churches or common houses and farms. Marble masterpieces become powder used as cement… temples and buildings symbolizing the evil misbelieves of the past were exploited of their materials and first of all of their metal.
In the coliseum construction iron bars had been inserted amidst the travertine blocks with the aim of securing those one over the other. The holes you see today are a result of this research of metal that occurred between the 6th and 8th century. Archeologist estimate the amount of metal spoiled is about 200 tons.