During the second century AD, Rome’s population reached 1.5 million people which made it the most populated city of the entire world. At the End of the Roman Empire, the city was nearly abandoned. Most buildings started to collapse due to fire, flood, earthquake and pillaging, or were simply abandoned. Many times, new foundations were built on top of the ruins or previous buildings readapted on purpose and the level of the city grew throughout the centuries. Underneath the present street level, Rome has an incredible number of ancient crypts, churches, tombs, private houses, etc. Don’t miss the opportunity to visit the underground remains of the Basilica of San Clemente with its Mithraeum, the houses of the Caelian hill and Catacombs of the ancient Appian Way. Your walk through the levels of the Eternal City will be like nothing you have ever seen.