Two main passages have a series of symbols and carvings on their walls; the tombs found there are mostly loculi and arcosolium emplacements, or niches surmounted by a round arch intended to hold sarcophagi, typical of Roman catacombs. Within the catacomb complex of Santa Mustiola, a polysome arcosolium, that is, intended to accommodate the bodies of several deceased persons, stands out for its particularity.
The catacomb contains a crypt where there is an altar and numerous inscriptions, including one that refers to Lucius Petronius Dextrus, the first bishop of Chiusi, who died in the first half of the 4th century; the amount of epigraphic material found inside has led scholars to believe that the catacomb complex was used as a burial place for about one hundred and fifty years, from the third century until the early fifth century.
The catacomb would appear to be the site where St. Mustiola, now the patron saint of Chiusi, was originally interred and buried. The cult of St. Mustiola began in the 4th century. In the 5th century a basilica was dedicated to her, which was destroyed and rebuilt several times until its final demolition during the 19th century. Her remains were transferred to the Co-cathedral of San Secondario, inside which it is possible to admire the cenotaph of the saint.