crypt, is the recently discovered vaulted fourth-century "Tomb of the Julii". (See below for some descriptions of tombs). Artworks Towers and narthex In the towers to either side of the facade are two clocks. The clock on the left has been operated electrically since 1931. Its oldest bell dates from 1288. One of the most important treasures of the basilica is a mosaic set above the central external door. Called the "Navicella", it is based on a design by Giotto (early 14th century) and represents a ship symbolizing the Christian Church. The mosaic is mostly a 17th-century copy of Giotto's original. At each end of the narthex is an equestrian figure, to the north Constantine the Great by Bernini (1670) and to the south Charlemagne by Cornacchini (18th century). Of the five portals from the narthex to the interior, three contain notable doors. The central portal has the Renaissance bronze door by Antonio Averulino (called Filarete) (1455), enlarged to fit the new space. The southern door, the Door of the Dead, was designed by 20th-century sculptor Giacomo Manzù and includes a portrait of Pope John XXIII kneeling before the crucified figure of Saint Peter. The northernmost door is the "Holy Door" which, by tradition, is walled-up with bricks, and opened only for holy years such as the Jubilee year by the Pope. The present door is bronze and was designed by Vico Consorti in 1950 and cast in Florence by the Ferdinando Marinelli Artistic Foundry. Above it are inscriptions commemorating the opening of the door: PAVLVS V PONT MAX ANNO XIII and GREGORIVS XIII PONT MAX. Recently installed commemorative plaques read above the door as follows: Older commemorative plaques are removed to make way for the new plaque when the holy door is opened and sealed. Nave On the first piers of the nave are two Holy Water basins held by pairs of cherubs each 2 metres high, commissioned by Pope Benedict XIII from designer Agostino Cornacchini and sculptor Francesco Moderati, (1720s). Along the floor of the nave are markers showing the comparative lengths of other churches, starting from the entrance. On the decorative pilasters of the piers of the nave are medallions with relief depicting 56 of the first popes. In niches between the pilasters of the nave are statues depicting 39 founders of religious orders. Set against the north east pier of the dome is a statue of