BENEATH modern Rome is a hidden city, as still as Rome is chaotic, as dark as Rome is luminous, with its own peculiar animals, powerful odors, frigid waters, and spectacular ancient remains. Explorers will find theaters, baths, stadia, imperial villas, apartment buildings, fire stations, and pagan temples -- even an enormous sundial that used an Egyptian obelisk as a pointer. Millions of people come to Rome each year in search of antiquity, and walk unsuspectingly across these buried treasures during their tours of the celebrated surface ruins. Though structures like the Pantheon and the Coliseum are certainly impressive, they represent only a small fraction of the ancient city, and wind, rain, and air pollutants have not treated them kindly over the years. Wrapped in a thick protective blanket of earth, Rome's subterranean structures have endured the incessant chiseling of people and elements far better. With persistence and the occasional help of a guide, a visitor can explore this underground realm, to discover bright windows on Roman history and clues to the evolution of the modern city long vanished from the surface.
Ancient Rome slipped from sight gradually, in a 2,500-year process of natural silting and intentional burial that was already well advanced in classical times. Roman architects frequently tore the roofs from old buildings and filled their interiors with dirt, to make solid foundations for new structures. They embedded earlier buildings in tremendous landfills that raised the ground level of the entire site by several yards. Sometimes they entombed whole neighborhoods in this way. After the Great Fire of A.D. 64 devastated two thirds of the city, Nero spread the debris over the wreckage of republican Rome and then reshaped the city to his liking. Later, during Rome's long, bleak Middle Ages, nature continued the interment. The population shrank to tiny pockets within the broad ring of the imperial walls, abandoning the ancient city to relentless erosion that wore away the uplands and redistributed them over low-lying areas. Roman buildings that remained exposed contributed significantly to the landfill. Archaeologists have estimated that the collapse of a one-story Roman house produced detritus six feet deep over its entire plan. Considering that Rome once boasted 40,000 apartment buildings, 1,800 palaces, and numerous giant public buildings, of which almost nothing survives, it is clear that the ancient city is buried under its own remains.
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.