Without being there, I can't imagine how great this underground city is, how deep it is, how it feels. The geological conditions and historical environment of the volcanic eruption have made the underground city a refuge for Christians to escape the pursuit of the Roman army in history, thus expanding the scale of the underground city. What we have visited is only a part of it that has been amazing. It is very hard to pass through narrow caves. Even in the low temperature of the underground city, people sweat all over, and sincerely marvel at the great nature and the wisdom of the ancients.