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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.

In the cellar of the massive, foursquare Palazzo della Cancelleria, in the heart of Rome, is a stretch of the Euripus, an ornamental canal that traversed this area, once a garden district. Now far belowground, it still brims with water, clear and unearthly blue. Writing from exile, a homesick Ovid fondly recalled the Euripus flowing between elegant lawns and porticoes. Ancient graffiti still visible beside the canal express less-elevated sentiments. "Scummy Ready-for-Anything gives it to her lovers all the time," an anonymous Roman penned in careful letters. "Crap well," another wrote just beside, either in response or as a general exhortation to passers-by.

Striking subterranea underlie the most ordinary scenes. A trapdoor in the courtyard of a bustling apartment complex on Via Taranto, not far from San Giovanni in Laterno, opens upon two perfect Roman graves, festooned with fresco grapevines and pomegranates, bewailed by red and blue tragic masks, guarded by mosaic goddesses. The nondescript palazzo at Via della VII Coorte 9, in the Trastevere district across the river, sits atop a complete Roman fire station, with its broad internal courtyard and central fountain, sleeping quarters, latrine, and shrine to the divinity who protected firemen. The busy train tracks on the eastern border of Porta Maggiore conceal a mysterious hall known as the Underground Basilica, apparently the temple of a first-century neo-Pythagorean cult. Handsome mosaic floors, three aisles, and a semicircular apse give it the look of a church, but stucco friezes on the walls show Orpheus leading Eurydice back from Hades, Heracles rescuing Hesione from the sea monster, and other scenes of mythological deliverance.

The grandest of all Roman subterranea lies beneath the shabby gardens on the eastern slopes of the Esquiline Hill, where homeless immigrants sleep and children play roughneck soccer against the startlingly big backdrop of the Coliseum. An entrance of crumbling brickwork leads down into the Golden House, a vast, megalomaniacal residence that Nero built atop ruins from the Great Fire; his successors, after damning Nero's memory, covered it with the Coliseum and other public buildings. An entire wing of the villa is buried here -- a labyrinth of corridors, vaulted chambers, and domed halls immersed in total darkness. Here and there a flashlight will illumine sections of the original Roman decoration: landscapes alive with mythological beasts and odd anthropomorphic figures. These frescoes attracted the greatest artists of the Renaissance, who clambered down with torches to sketch the drawings, hold merry picnics of apples, prosciutto, and wine, and scratch their names unselfconsciously into the plaster (many famous autographs, including Domenico Ghirlandaio, Martin van Heemskerck, and Filippino Lippi, are still visible). They emerged from these underground rooms -- "grottoes," as they called them -- to decorate Rome in a new, "grotesque" style.

Exploring Rome's subterranea, one learns certain rules of thumb. Low-lying areas like Trastevere, which millennia of floods have paved in heavy layers of silt, are rich in sites. Even better are zones that have been continuously inhabited since classical times (the Campo Marzio, for example, with the Pantheon at its center), where subterranea have escaped the violence of deep modern foundations. For much the same reason churches make excellent hunting. In many crypts and side chapels are shadowy locked doorways that the sacristan can often be persuaded to open, for a modest contribution. They lead down to Roman baths, taverns, prisons, military barracks, brothels, and other remains. Pagan temples are especially common, perhaps because Christian builders wanted to occupy and eradicate the sacred places of competing religions. Beneath the polished marble floors of San Clemente, Santa Prisca, Santo Stefano Rotondo, and several other churches are shrines to Mithras, an Iranian god of truth and salvation who was one of Jesus' main rivals during the later empire. These snug, low-roofed halls are flanked by benches where the worshippers reclined, with a niche at the far end for the cult statue: a heroic young Mithras in a flowing cape, plunging his sword into the neck of an enormous bull. By the warm light of torches all-male congregations once worshipped Mithras here in strange rites of water and blood, vaguely suggested in graffiti still visible beneath Santa Prisca: "Sweet are the livers of the birds, but worry reigns." "And you redeemed us by shedding the eternal blood."

FOR some Romans the hidden city beneath their feet has become an obsession. The photographer Carlo Pavia, lean and intense, has for the past twenty years rappelled down into ancient mines and apartment houses, scuba-dived in underground halls filled with icy groundwater, and pulled on hip waders and a gas mask and slogged back into the Cloaca Maxima, an ancient sewer that winds its way beneath much of Rome. He describes unearthly scenes: colonies of fat albino worms; rats as big as lapdogs; African and Arabian plants flourishing in the rooms beneath the Coliseum, grown from seeds fallen from the coats of exotic animals imported by the Romans for their entertainments. Packs of saltericchi, a kind of jumping spider, rove the deepest, most humid recesses. "At the first sign of light they panic and start hopping around," Pavia explains. "I have to move carefully, shooing them ahead of me with my lamp." Pavia recently founded a magazine, Forma Urbis, that each month illustrates selected sites with his outstanding photographs.

Other subterraneophiles are less athletic but equally obsessed. Emanuele Gatti is a round, jovial retiree who has devoted much of his life to underground Rome. As a government archaeologist he oversaw more than thirty years' worth of construction projects in the historic center, and he has fleshed out his experiences with painstaking archival research to produce a detailed map of ancient remains -- a kind of x-ray that lays a faint modern city over the sharp, clear bones of its subterranea. He runs his hand over the sea of symbols and annotations that is his magnum opus, eagerly indicating points of contact between the two worlds. "See here how the façade of the Parliament building rests directly on the façade of Alexander's Baths? Ancient walls still support modern buildings like this throughout the city. They are still 'alive,' you might say." Gatti hopes that some of the billions of dollars to be spent beautifying Rome for the Great Jubilee in the year 2000, a twelve-month festival of the Catholic Church that may bring some 40 million additional visitors to the city, will help to preserve the underground city and make it more accessible.

A few people are working on accessibility already. Three years ago Bartolomeo Mazzotta, then a graduate student in archaeology, assembled a handful of fellow experts to form Itinera, one of several new tour services that specialize in underground Rome. These services provide the best way to explore many subterranea, presenting a detailed introduction to the history and archaeology of the sites and supplying government permits that are difficult for individuals to arrange. For a modest fee you join a group of ten to twenty on a visit that lasts about an hour. Though the commentary is normally in Italian, most guides can field questions in English as well. Veteran visitors bring a flashlight, wear sturdy shoes that will give good traction on wet ground, and drape a sweater or shawl over their shoulders, as subterranea are often chilly even in the summer.

Most of the tour services schedule their visits months in advance and have a devoted following, so it is a good idea to book by telephone at least two weeks ahead. The best ones, such as Itinera (011-396-275-7323) and LU.PA. (396-519-3570), are run by trained archaeologists with years of experience belowground. Other good choices include Genti e Paesi (396-8530-1755) and Città Nascosta (396-321-6059), which generally take a more historical or art-historical approach. All these will arrange custom tours of multiple sites for groups. A complete listing of scheduled visits appears each week in Romac'è, a booklet available at newsstands in Rome and on the World Wide Web (http://www. villedit.it). Beyond specific tours he leads, an expert like Mazzotta is a gold mine of information about the best parts of underground Rome to visit, which sites are closed for renovation, and which can be seen without a permit. Mazzotta explains that most tour participants are Romans, who are increasingly eager to explore the lower city. He says, "Roma sotterranea is becoming a real cult."

In fact it is a very old cult, though some of its most ardent believers prefer to remain anonymous. Houses and workshops in the older neighborhoods of Rome frequently perch atop ancient remains, which here and there jab stone fingers up through the surface, just as Montaigne witnessed four centuries ago: massive granite columns sprouting from basement floors, Roman brick archways ridging foundation walls. The inhabitants, often elderly Romans whose families have lived in the same buildings for generations, may guard their secret subterranea carefully, fearing eviction by government authorities if word gets out. Gain their trust, however, and they will show off their underground treasures with great pride. They tell of other subterranea -- deep tunnels that traverse the city, vast and mysterious sanctuaries and palaces, a realm of oral tradition somewhere between science and legend. These elderly Romans are acutely aware of the lower city beneath their surface lives. Rome, they say, is haunted by its subterranea.

Photographs by Carlo Pavia