For some reason, catacombs tend to carry a negative or at least creepy connotation in society today. There have been catacombs in existence for thousands of years. The most widely known catacombs are in Rome. Coming in a close second are the catacombs of Paris. These are large and extensive catacombs that are among the earliest and largest.
Catacombs are man-made caves.
Some early catacombs started out as real caves. However, as people began to see a value in these caves as a repository for corpses, mankind started to create their own caves for this purpose. Thousands of feet of underground caves snake their way beneath Paris and Rome.
Once catacombs became a way of burial, people would enlarge them to keep using them for a place to bury the dead.
The oldest and best known catacombs show signs of having been enlarged numerous times. Additional chambers were created to hold more and more bodies. New tunnels were excavated as paths off of large existing tunnels to open up new underground passages.
The walls were often hollowed out to make individual burial plots.
Some of these were simply lines of single body openings. Others were small rooms designed hold the dead from a specific family or group. Artifacts were entombed with the dead to bring comfort after death or to signify social rank or to serve some other purpose.
Catacombs are frequently linked to early Christianity.
This link comes from two directions. On the one hand, Christians were buried there in secret places to prevent the bodies being vandalized. Early Christians also used the catacombs as meeting places. Because many people feared going into these caves where the dead were stored, Christians could worship freely without much fear of discovery from those who persecuted the early church.
Some catacombs became underground burial places beneath churches.
When the persecution of the church subsided somewhat, churches would dig out caves or tunnels under the church. In this way, the dead could be buried in "hollowed" ground.
The catacombs were excellent places for burial.
Because of their depth, those buried in catacombs rarely ever became lifted back to the surface by flooding or other natural events. The catacombs also help prevent the waste from decaying bodies from reaching and polluting the ground water. In drier climates, natural mummification also occurred to further preserve the bodies for year.