In ancient times, the Circus of Nero, where St. Peter is said to have been crucified, was slightly to the left of where the basilica is now located. Peter was buried here in A.D. 64 near the site of his execution, and in 324 Constantine commissioned a basilica to be built over Peter's tomb. That structure stood for more than 1,000 years until it verged on collapse. The present basilica, mostly completed in the 1500s and 1600s, is predominantly High Renaissance and baroque. Inside, the massive scale showcases some of Italy's greatest artists: Bramante, Raphael, Michelangelo, and Maderno. In a church of such grandeur -- overwhelming in its detail of gilt, marble, and mosaic -- you can't expect much subtlety. It's meant to be overpowering.

In the nave on the right (the first chapel) stands one of the Vatican's greatest treasures: Michelangelo's exquisite Pietà, created while the master was still in his early 20s but clearly showing his genius for capturing the human form. (The sculpture has been kept behind reinforced glass since a madman's act of vandalism in the 1970s.) Note the incredibly lifelike folds of Mary's robes and her youthful features (although she would've been middle-aged at the time of the Crucifixion, Michelangelo portrayed her as a young woman to convey her purity).

Much farther on, in the right wing of the transept near the Chapel of St. Michael, rests Canova's neoclassical sculpture of Pope Clement XIII. The truly devout stop to kiss the feet of the 13th-century bronze of St. Peter, attributed to Arnolfo di Cambio (at the far reaches of the nave, against a corner pillar on the right). Under Michelangelo's dome is the celebrated twisty-columned baldacchino (1524), by Bernini, resting over the papal altar. The 29m-high (96-ft.), ultra-fancy canopy was created in part, so it's said, from bronze stripped from the Pantheon, although that's up for debate.

In addition, you can visit the Treasury, which is filled with jewel-studded chalices, reliquaries, and copes. One robe worn by Pius XII strikes a simple note in these halls of elegance. The sacristy contains a Historical Museum (Museo Storico) displaying Vatican treasures, including the large 1400s bronze tomb of Pope Sixtus V by Antonio Pollaiuolo and several antique chalices.

You can also head downstairs to the Vatican grottoes, with their tombs of the popes, both ancient and modern (Pope John XXIII gets the most adulation). Behind a wall of glass is the tomb of St. Peter himself.

To go even farther down to the Vatican necropolis, the area around St. Peter's tomb, you must apply well in advance at the Ufficio Scavi (tel. 06-69885318, scavi@fsp.va), through the arch to the left of the stairs up the basilica. You specify your name, the number in your party, your language, and dates you'd like to visit. They'll notify you by phone or e-mail of your admission date and time. For 10€, you'll take a guided tour of the tombs that were excavated in the 1940s, 7m (23 ft.) beneath the church floor. For details, check www.vatican.va.

After you leave the grottoes, you'll find yourself in a courtyard and ticket line for the grandest sight: the climb to Michelangelo's dome, about 113m (375 ft.) high. You can walk up all the steps or take the elevator as far as it goes. The elevator saves you 171 steps, and you'll still have 320 to go after getting off. After you've made it to the top, you'll have an astounding view over the rooftops of Rome and even the Vatican Gardens and papal apartments -- a photo op, if ever there was one.

A St. Peter's Warning -- St. Peter's has a strict dress code: no shorts, no skirts above the knee, and no bare shoulders. You will not be let in if you don't come dressed appropriately. In a pinch, men and women alike can buy a big, cheap scarf from a nearby souvenir stand and wrap it around their legs as a long skirt or throw it over their shoulders as a shawl. If you're still showing too much skin, a guard hands out blue paper capes similar to what you wear in a doctor's office. No photographs are allowed.