The Catacombs of Rome are ancient catacombs, or unerground burial places under or near Rome, Italy, of which there are at least forty, some discovered only in recent decades. Though most famous for Christian burials, they include pagan and Jewish burials, either in separate catacombs or mixed together. They began in the 2nd century, as much as a response to overcrowding and shortage of land as a need for persecuted Christians to bury their dead secretly, although that was a factor. The soft volcanic tufo rock under Rome is highly suitable for tunnelling, as it is softer when first exposed to air, hardening afterwards. Many have kilometres of tunnels, in up to four storeys or layers.