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.