About the Catacombs of the Saints Marcellino and Pietro:

Thanks to laser technology, the frescoes have been restored to their original beauty and finally re-opened to the public in 2014.
The site is next door to the ruins of the Mausoleum of Helena (mother of Emperor Constantine) which was built by Constantine in about 315 A.D.
Tours are available. You need to book in advance ( see telephone number below) and there is a small cost. Like many places in Rome, they are closed the entire month of August.
Finding the Catacombs of Saints Marcellino and Pietro:
The Catacombs of Sts. Marcellinus and Peter are located on the Via Casilina. They are easily reached via the tram line that runs from the Stazione per le Linee del Lazio, located on the west side of the Roma Termini station to the Berardi station that is across the street from the catacomb complex.
Address: Via Casilina, 641 – 00177 ROMA – Italy
Tel: +39 339 65 28 887 (Info&Booking)
email: santimarcellinoepietro@gmail.com