The Vatican is marking the 500th anniversary of St. Peter's Basilica with an exhibit featuring Michelangelo's model of the church dome and an ancient inscription reading "Peter is here" — a reference to the tomb of the apostle Peter on which the basilica is believed built.

This year also marks the 500th anniversaries of the formation of the Swiss Guards and the creation of the Vatican Museums. But officials said Thursday that the laying of the first stone of St. Peter's was by far the most important celebration.

"No other monument in the world has had the contribution of such celebrated architects as the new St. Peter's Basilica," said Cardinal Francesco Marchisano, archpriest of the basilica, citing Michelangelo, Bernini, Raphael and others.

Pope Julius II laid the first stone of St. Peter's on April 18, 1506, erecting the new church over one built in the fourth century by Constantine, the Roman emperor who was a strong supporter of Christianity.

Monsignor Angelo Comastri, who is in charge of the basilica's upkeep, told a news conference there was a clear reason why a new site wasn't chosen for the basilica, which Julius had wanted to serve as a tangible image of the greatness and power of the church.

"They didn't want to move by even a centimeter from the Vatican hill because ancient tradition affirmed that this was the place of the tomb of the apostle Peter," he said.

During excavations undertaken by Pope Pius XII in the 1900s, workers discovered a wall with a block of Greek graffiti scrawled on it "Petros eni" — "Peter is here" — that Marchisano said dated from 180 to 200 A.D.

The block of graffiti is one of the highlights of the exhibit "Petros Eni" which opens Oct. 3 and runs through March 8, 2007, at the Vatican. Other highlights include a pre-Constantine sarcophagus with the figure of someone praying on it that was unearthed in 2003, as well as the wooden model made by Michelangelo of the basilica's dome.

Michelangelo was given the job of designing the church in 1546 at age 71, one of at least 10 architects who were commissioned over the basilica's 120-year construction. Realizing that he wouldn't see the project completed, the artist built a huge wooden model, hoping that his vision would be respected after death.

Once completed, the basilica became the largest Roman Catholic church in the world at the time, 610 feet long and 449 feet at its widest. Today, visitors can look for lines on the marble floor of the central nave that mark out the length of other churches around the world.

The basilica has 5,000-20,000 visitors a day, though the number can swell to 30,000. Marchisano said the small number of people allowed into the necropolis underneath the basilica — 50,000 a year — was due to the limited space and the need to protect the ancient paintings and relics from exposure to too much humidity.