They were openings for the various trap doors and ramps that raised animals and other entertainments to the Colosseum's "stage floor." The hundreds upon thousands of additional holes, on the other hand, were caused by employees pilfering the iron to use on other projects or for armament. These thieves would break into the mines where the metal was stored and steal as much as they could carry. The Colosseum was originally built to hold wild animals, but after many years of theft it became clear that this was not a problem for which you should go to jail. So the Roman government decided to sell the metal from the stolen cannons and swords and pay for other people to build more coliseums. Unfortunately, this only made things worse because now there were no guards at the mine entrances! You might think that stealing metal would not be so bad, but imagine how much money it must have cost! The mine workers probably didn't get paid very well, if at all. And even if they did manage to sell some of the metal, they would still need food and shelter. That means working even longer hours for less money, which would just make them want to steal even more. In conclusion, theft was very common in ancient Rome because there was no police force and no system of justice. The only way to resolve disputes was through violence.