Talk about the "underground," and a growing legion of Romans will excitedly take up the story, offering tidbits about where to go, who to talk to, what's been seen, and what's allegedly awaiting discovery around the next bend in the sewer. The sewer? That's right. Roma Sotteranea (Subterranean Rome) is neither a subway nor a trendy arts movement, but the vast historic ruins of a city that has been occupied for nearly 3,000 years, the first 2 millennia of which are now largely buried by natural sediment and artificial landfills. Archaeologists estimate that these processes have left the streets of ancient Rome as much as 20 yards beneath the surface.

Consider this: Each year, an inch of dust in the form of pollen, leaves, pollution, sand, and silt from disintegrating ruins settles over Rome. That silt has really taken a toll in its own right. Archaeologists estimate that the ruins of a one-story Roman house will produce debris 6 feet deep over its entire floor plan. When you multiply that by more than 40,000 apartment buildings, 1,800 palaces, and numerous giant public buildings, a real picture of the burial of the ancient city presents itself. You should also take note of the centuries-old Roman tradition of burying old buildings in landfills, which can raise the level of the earth up to several yards all at once. In fact, past builders have often filled up massive stone ruins with dirt or dug down through previous landfills to the columns and vaults of underlying structures, and then laid a foundation for a new layer of Roman architecture.

As a result, many buildings on the streets today actually provide direct access to Rome's inner world. Doorways lead down to hidden crypts and shrines--the existence of which are closely guarded secrets. Nondescript locked doors in churches and other public buildings often open on whole blocks of the ancient city, streets still intact. For example, take San Clemente, the 12th-century basilica east of the Colosseum, where a staircase in the sacristy leads down to the original 4th-century church. Not only that, but a staircase near the apse goes down to an earlier Roman apartment building and temple, which in turn leads down to a giant public building dating back to the Great Fire (A.D. 64). Another interesting doorway to the past is in the south exterior wall of St. Peter's, leading down to an intact necropolis. That crumbling brick entry in the gardens on the east side of Esquiline Hill carries you into the vast Domus Aurea (Golden House), Nero's residence, built on the ruins left by the Great Fire.

Don't expect a road map of this subterranean world; it's a meandering labyrinth beneath the streets. A guided tour can be useful, especially those focusing on Roman excavations and anything to do with church crypts. Several tour companies now offer selected subterranean views, lasting 90 to 120 minutes and costing 12.50€ to 25€ ($11-$22). The best are provided by Itinera (tel. 06-278-00785 ) and LUPA (tel. 06-574-1974 ), both run by trained archaeologists. Città Nascosta (tel. 06-321-6059 ) offers offbeat tours to less-visited churches and monuments, and advertises the week's schedule via a recorded phone announcement that changes every week.