Known as "the little Vatican," this crypt is the burial site of 16 popes and several martyrs from the 3rd century. Original Greek inscriptions on the walls display the names of five popes, including Pontianus, Antherus, Fabian, Lucius, and Eutichian. Four tombstones bear the title of "bishop," representing the Pope's role as head of the Church. Pope Sixtus II, a martyr persecuted by Emperor Valerian, rests on the front wall.