The physical proportions of the sublime St. Peter's Basilica are staggering: it covers about 18,100 square yards, extends 212 yards in length, and carries a dome that rises 435 feet and measures 138 feet across its base. Its history is equally impressive: no fewer than five of Italy's greatest artists -- Donato Bramante (1444-1514), Raphael (1483-1520), Baldassare Peruzzi (1481-1536), Antonio Sangallo the Younger (1483-1546), and Michelangelo (1475-1564) -- died while working on the construction of this version of St. Peter's.

The history of the original St. Peter's goes back to the year AD 319, when the emperor Constantine built a basilica over the site of the tomb of St. Peter (died AD 64). This early church stood for more than 1,000 years, undergoing a number of restorations, until it was on the verge of collapse. Reconstruction began in 1452 but was abandoned for lack of funds. In 1506 Pope Julius II (1443-1513) instructed the architect Bramante to raze the existing structure and build a new and greater basilica. In 1546 Pope Paul III persuaded the aging Michelangelo to take on the job of completing the building. Revising Bramante's plan, Michelangelo designed the dome to cover the crossing, but his plans, too, were modified after his death. The cupola, one of the most beautiful in the world, was completed by Giacomo della Porta (circa 1537-1602) and Domenico Fontana (1543-1607). The new church wasn't completed and dedicated until 1626; by that time the ground plan had shifted from a Greek cross to a Latin one, creating a longer nave but obscuring the view of the dome from the piazza. Under the portico, 15th-century bronze doors by Filarete (circa 1400-1469), salvaged from the old basilica, fill the central portal. Off the entry portico, Bernini's famous Scala Regia, the ceremonial entryway to the Vatican Palace and one of the most magnificent staircases in the world, is graced with Bernini's dramatic statue of Constantine the Great.

The cherubs over the holy-water fonts will give you a sense of the grand scale of St. Peter's: the sole of each cherub's foot is as long as the distance from your fingertips to your elbow. It is because the proportions of this giant building are in such perfect harmony that its vastness may escape you at first. But in its megascale -- inspired by the size of the ancient Roman ruins -- it reflects Roman grandiosità in all its majesty.

Over an altar in a side chapel near the entrance is Michelangelo's Pietà. Legend has it that the artist, only 22 at the time the work was completed, overheard passersby expressing skepticism that such a young man could have executed such a sophisticated and moving piece. Offended at the implication, he crept back that night and signed the piece -- in big letters, on a ribbon falling from the Virgin's left shoulder across her breast, where no one could possibly miss it. His name is there today.

Four massive piers support the dome at the crossing, where the mighty Bernini baldacchino (canopy) rises high above the papal altar. "What the barbarians didn't do, the Barberini did," 17th-century wags quipped when Barberini Pope Urban VIII had the bronze stripped from the Pantheon's portico and melted down to make the baldacchino (using what was left over for cannonballs). The pope celebrates mass here, over the grottoes holding the tombs of many of his predecessors. Deep in the excavations under the foundations of the original basilica is what is believed to be the tomb of St. Peter. The bronze throne behind the main altar in the apse, the Cathedra Petri (Chair of St. Peter), is Bernini's work (1656), and it covers a wooden-and-ivory chair that St. Peter himself is said to have used. However, scholars contend that this throne probably dates only from the Middle Ages. See how the adoration of a million lips has completely worn down the bronze on the right foot of the statue of St. Peter in front of the near right pillar in the transept.

The entrance to the Grotte Vaticane (Vatican Grottoes), which hold the tombs of many popes, is at the crossing. The only exit from the grottoes leads outside St. Peter's, to the courtyard that holds the entrance to the dome.; OPEN: Apr.-Sept., daily 7-6; Oct.-Mar., daily 7-5.

A small but rich collection of Vatican treasures is housed in the Museo Storico in the Sacristy, among them precious antique chalices and the massive 15th-century sculptured bronze tomb of Pope Sixtus V (1520-90) by Antonio del Pollaiuolo (1431-98).; COST: EUR9; OPEN: Daily 8-5.

The roof of the church, reached by elevator or stairs, is a landscape of domes and towers. A short interior staircase leads to the base of the dome for a dove's-eye view of the interior of St. Peter's. Then, only if you are stout of heart and sound of lung should you attempt the taxing and claustrophobic climb up the narrow stairs -- there's no turning back! -- to the balcony of the lantern, where the view embraces the Vatican Gardens and all of Rome. PHONE: 06/69883462; COST: Elevator EUR5, stairs EUR4; OPEN: Daily 8-5. Closed during ceremonies in the piazza. COST: Free.