By 1580, when Montaigne visited Rome, the classical city was all but invisible. He observed that when modern Romans dug into the ground, they frequently struck the capitals of tall columns still standing far below. "They do not seek any other foundations for their houses than old ruined buildings or vaults, such as are seen at the bottom of all the cellars." Impressed by the spectacle of the triumphal arches of the Forum rising from deep in the earth, he noted, "It is easy to see that many [ancient] streets are more than thirty feet below those of today." Even now the burial process continues. Each year an inch of dust falls on Rome, composed of leaves, pollution, sand from the nearby seacoast, and a stream of powder from hundreds of ruins dissolving steadily in the wind. In certain places we are more than ten yards farther from ancient Rome than Montaigne was.

A GOOD place to begin exploring Rome's layers is San Clemente, a twelfth-century basilica just east of the Coliseum. Descend the staircase in the sacristy and you find yourself in a rectangular hall decorated with fading frescoes and greenish marbles, lit by sparse bulbs strung up by the excavators. This is the original, fourth-century San Clemente, one of Rome's first churches. It was condemned around A.D. 1100 and packed full of earth, Roman-style, as a platform for the present basilica. A narrow stair near the apse of this lower church leads down to the first-century structures upon which it, in turn, was built: a Roman apartment house and a small temple. The light is thinner here; cresses and fungi patch the dark brick and grow delicate halos on the walls behind the bare bulbs. Deeper still, on the fourth level, are several rooms from an enormous public building that was apparently destroyed in the Great Fire and then buried by Nero's architects. At about a dozen yards belowground the massive tufa blocks and herringbone brickwork are slick with humidity, and everywhere is the sound of water, flowing in original Roman pipes. No one has excavated below this level, but something is there, for the tufa walls run another twenty feet or so down into the earth. Something is buried beneath everything in Rome.

Most major landmarks, in fact, rest on construction that leads far back into the past. Tucked under Michelangelo's salmon-pink Senatorial Palace on the Capitoline Hill is a tidy little temple to Veiovis, a youthful Jove of the underworld, among the most ancient gods of the Roman pantheon. Beneath the sanctuary excavators have found traces of a still-earlier shrine. A small passageway in the south exterior wall of St. Peter's Basilica leads into an eerily intact Roman necropolis that underlies the entire center aisle. The passage becomes the main street of a miniature city of the dead, fronted by ornate two-story mausoleums on which Christ and the Apostles stand alongside Apollo, Isis, Bacchus, and rampaging satyrs. This necropolis first came to light in the Renaissance, when the basilica was rebuilt: pontiffs and architects watched in horror as an endless stream of pagan relics issued from the floor of Catholicism's most sacred church.