Etruscans used to bury their dead in underground chambers. Christians recreated the practice because they did not want to cremate their dead due to their belief in bodily resurrection. Hence they began to bury their dead, first to simple graves and sometimes in burial vaults of pro-Christian patricians.
First large-scale catacombs were built from 2nd century onwards. Originally they were carved through soft rock outside the boundaries of the city because Roman law forbade burial places within city limits. At first they were used both for burial and the memorial services and celebrations of the anniversaries of Christian martyrs (following a similar Roman customs). They probably were not used for regular worship. They might have been used as hiding places in times of persecution but the matter is controversial.
There are forty known subterranean burial chambers in Rome. They were built along Roman roads, like the Via Appia, the Via Ostiensis, the Via Labicana, the Via Tiburtina, and the Via Nomentana. Names of the catabombs – like St Calixtus and St Sebastian alongside Via Appia – refer to martyrs that are supposedly buried there.
Christian excavators built vast systems of galleries and passages on top of each other. They lie 7-19 meters (22-65 ft) below the surface in area of more than 240 hectares (600 acres). Narrow steps that descend as many as four stories join the levels. Passages are about 2.5x1 meters (8x3 feet). Burial niches were carved into walls. They are 40-60 cm (16-24 in) high and 120-150 cm (47-59 in) long. Bodies were placed in chambers in stone sarcophagi in their clothes and bound in linen. Then the chamber was sealed with a slab bearing the name, age and the day of death. Fresco decorations were typically Roman. The catacomb of Saint Agnese is a small church.
In 313 AD, Christianity became a state religion. At first many desired to be buried in chambers alongside martyrs. However, the practice of catacomb burial declined slowly and the dead were increasingly buried in church cemeteries. In the 500s catacombs were used only for martyrs’ memorial services. Apparently Goths, Vandals and Lombards that sacked Rome also violated the catacombs, possibly looking for valuables. By 900s catacombs were practically abandoned and holy relics were transferred to above-ground basilicas. In intervening centuries they remained forgotten until they were accidentally rediscovered in 1578.
Currently maintenance of the catacombs is in the hands of the papacy.
There are also Jewish catacombs in Rome.