A Catacomb is an underground burial place, while a Hypogeum is an underground vault or chamber, often hosting Catacombs.
The Maltese islands contain many such ancient cemeteries, the majority of which have been attributed to the second half of Roman Era, and are mostly Paleo-Christian.
The Bible suggests that the shipwreck of St. Paul in 60AD led to the first local proclamation of the Christian Faith.
Malta being a hub at the crossroads of the Mediterranean Sea implies that further knowledge of this creed would be brought along with traders and travellers over time.
Archeology shows that by 200 AD, Christianity had not only become formally organised but also the main religion of the islands.
Christians chose to inter their dead rather than cremate them and duly moved away from typical Roman methods and rites. Some pagan or pre-Christian habits were however maintained, such as the placing of pottery and items that might be useful for the journey into the afterlife within tombs.
The local transition to this growing religion seems to have evolved rather naturally and peacefully, but there was cause of conflict due to Roman law and the deity status attributed towards their Emperor.
The eventual persecution of Christians in Rome and other regions of the Empire was another grave concern.
According to historian Harrison Lewis, this could have led to Maltese Christians seeking out various remote places where they could bury their dead without fear or interference. (1)
They often adopted existing caves or dug chambers and tunnels in hillsides, some developing into impressive extensive networks.
A very important example may be found at Ta' Bistra, not far from the marble plaque commemorating the construction of the Victoria Lines.
There are others within the grounds of Fort Mosta and underneath one of its external bastions
More catacombs are documented close to Torre Cumbo, at Tal-Ghammariet and several others may be found just out of Mosta at Bingemma, San Pawl Milqi, and Salina.
Notes & References:
Ancient Malta by Harrison Lewis
Note1: Traditionally, it was believed that Christians used catacombs to escape Roman persecution as could have been the case in Rome itself. There is no record however of local persecution. A comprehensive study of the local Roman settlement network shows that most catacombs were not actually situated in remote places but in the proximity of structures or habitations.