Story of Catacombs:

Catacombs are underground burial places. In the early days of Christianity, due to the persecutions, the Church had to go "underground". Because the belief of "communion of saints", tombs of deceased believers became an ideal people for gathering and worship. So early church world organized it as "burial association" which provided free burial services for anyone.

At the beginning, the government was very happy about it. They did not need to worry about dead people on the street any more (which was pretty common in early human history). The "burial association" would take care of it for free.

Then they realized what was happening "underground".

The policy changed to:
whoever found gathering underground without permissions would be sent to jail. Then…

Six Popes were martyred for them. Among them, Pope St. Sixtus II was caught in the middle of performing Mass. His head was cut off on the altar right there.