The great dome soars above the altar and the baldacchino, sumptuously embellished with mosaic and stucco ornaments. It is supported by four structural piers with a perimeter of 71 m. and a height of 120 m. from the ground to the roof of the lantern.
Many artists worked on these decorations. Clement VIII commissioned Giuseppe Cesari, known as Cavalier d'Arpino (1568-1640) to carry out the upper part of the decoration. He prepared the cartoons from 1603 to 1612. His drawings were simultaneously transposed into mosaic by the best mosaic artists of the period.