First, stop at the Catacombs of St. Sebastianus, a huge site of 12 km of galleries in total in which St. Sebastianus and many Christians were buried. These quarries are the origin of a pagan cemetery, later used by Christians. Then the term "catacumbas" ended up qualifying all the underground cemeteries, which were simply called "catacombs".
In alternative you'll visit the Catacombs of St. Callixtus in which were buried tens of martyrs, 16 Popes and many Christians.
Afterwards, you will visit the Basilica and Catacombs of Saint Clemente. Admire the spectacular mosaics and frescoes, travel back in time to explore the Basilica of the Fourth Century, then descend into the world of the First Century where there is still a pagan temple.
Finally, you'll visit the Cripta dei Cappuccini located just under the Church Santa Maria della Concezione, which is considered a true work of art for its decorations executed with human bones by an unknown "brilliant artist".