Christianity was outlawed by the Emperor Nero and Christians were persecuted. Many thousands were killed. They had to hold religious services in secret,so they gathered in the undergound tombs. For a time,they were safe from attack there, from about 250 AD,even the burial grounds were not safe. The Christians then blocked up the entrancesand made secret ones. The tombs stretched for miles in the underground galleries, like an enormous mine. They were reached by steps often as deep as 12 meters under the ground. It is said that they would be longer than the whole of Italy if the passages were in a straight line!