Complex sentence: In the 4th century a small Christian community lived in Pianosa and left traces of their presence in since excavated catacombs that are on two levels and are the largest north of Rome; 700 catacombs were discovered which indicates a fair numbers of residents.
Split into simpler sentences: In the 4th century a small Christian community lived in Pianosa and left traces of their presence in since excavated catacombs. These are on two levels and are the largest north of Rome; 700 catacombs were discovered, indicating a fair number of residents.