Directly to the east of the church is St Peter's Square (Piazza di San Pietro), built by Gian Lorenzo Bernini between 1656 and 1667. It is surrounded by an elliptical colonnade with two pairs of Doric columns which form its breadth, each bearing Ionic entablatures. This is an excellent example of Baroque architecture, where creativity is coupled with flexible guidelines. In the center of the colonnade is a 25.5 metre (83.6 ft) tall obelisk. Domenico Fontana finished moving the obelisk to its present location on September 28, 1586 by order of Pope Sixtus V. The obelisk dates back to the 13th century BC in Egypt, and was moved to Rome in AD 37 to stand in the Circus of Nero some 250 metres (820 ft) away. Including the cross on top and its base, the obelisk reaches 40 metres (131 ft). The Vatican obelisk is notable for being the second largest standing obelisk and the only one that remained standing since it was erected. On top of the obelisk there used to be a large bronze globe allegedly containing the ashes of Julius Caesar. The original bronze globe was removed when the obelisk was re-erected in St Peter's Square by Domenico Fontana. There are also two fountains in the square, the north one by Maderno (1613) and the southern one by Bernini (1675). The square is reached mainly through the Via della Conciliazione built by Mussolini after the conclusion of the Lateran Treaties.
There is a widespread assumption that the dome, or cupola, as it presently stands, was designed by Michelangelo, who became chief architect in 1546. In fact, Michelangelo's design called for a spherical dome. At the time of his death (1564), only the drum, the base on which a dome rests, had been completed. The dome proper was redesigned and vaulted, between 1585 and 1590, by the architect Giacomo della Porta, with the assistance of Domenico Fontana, who was probably the best engineer of the day. Fontana built the lantern the following year, and the ball was placed in 1593.
As built, the double dome is brick, 42.3 metres (138.8 ft) in interior diameter (almost as large as the Pantheon), rising to 120 metres (394 ft) above the floor. In the mid-18th century, cracks appeared in the dome, so four iron chains were installed between the two shells to bind it, like the rings that keep a barrel from bursting. (Visitors who climb the spiral stairs between the dome shells can glimpse them.) The four piers of the crossing that support it are each 18 metres (59 ft) across. It is not simply its vast scale (136.57 m or 448.06 ft) from the floor of the church to the top of the added cross) that makes it extraordinary. Della Porta's dome is not a hemisphere, but a paraboloid: it has a vertical thrust, which is made more emphatic by the bold ribbing that springs from the paired Corinthian columns, which appear to be part of the drum, but which stand away from it like buttresses, to absorb the outward thrust of the dome's weight. The grand arched openings just visible in the illustration but normally invisible to viewers below, enable access (but not to the public) all around the base of the drum; they are dwarfed by the monumental scale of their surroundings. Above, the vaulted dome rises to Fontana's two-stage lantern, capped with a spire.
The egg-shaped dome exerts less outward thrust than a lower hemispheric one (such as Mansart's at Les Invalides) would have done. The dome conceived by Donato Bramante at the outset in 1503 was planned to be carried out with a single masonry shell, a plan discovered to be infeasible. San Gallo came up with the double shell, and Michelangelo improved upon it. The piers at the crossing, which were the first masonry to be laid, and which were intended to support the original dome, were a constant concern, too slender in Bramante's plan, they were redesigned several times as the dome plans evolved.