Athens is one of the largest, busiest, noisiest, and, in the eyes of many, ugliest cities on the Mediterranean. Yet in the center of this urban insanity, high above the bustle and cacophany, glistening even through the ochre cloud of pollution that envelops the city, sits the Acropolis, a stone hill topped with serene and graceful monuments, almost like a mirage.

To wind one's way along the slippery stone paths up to the top of the rocky promontory is to be gradually awakened to just how extraordinary was this accomplishment of the ancient Greeks.

From afar, stark and brilliant against the sky, the Parthenon dominates the city without weighing upon it. From up close, the monument rises huge and solid, radiating in vanilla brilliance, but losing none of its impression of lightness.

This mingling of strength, grace, and levity stuns the visitor with an overwhelming sense of power - the power of the minds of the men who conceived and built the monuments over 2,000 years ago, the power of will and imagination of the civilization that could conceive of such a vast project at a time when the city itself was little more than a large town and when the muscles and sweat of man were the basic tools.

It was in 448 BC when Athens' ruler Pericles asked his friend, the master sculptor Phidias, to direct and assist in the construction of the buildings whose ruins still sit atop the Acropolis.

Ten years later, the artist and his many assistants - architects, artists, artisans, masons, and laborers - put the finishing touches on the Parthenon, a work of art of incomparable refinement and perfection. Athens had reached a golden age and had only recently transformed the Greek city-states into an empire.

The monumental buildings on the Acropolis, principally the Parthenon, the Erechtheum, and Propylaea, symbolized for Athenians - and demonstrated to outsiders - the brilliance of the city's civilization, the possibilities of its wealth and power, and the superiority of its political creation - democracy.

Those days passed long ago, but the monuments on the Acropolis, in particular the Parthenon, have remained a powerful reminder of past glory and a constant source of national pride. Over the centuries, city planners have ensured that surrounding neighborhoods had at least one unobstructed view of the Acropolis.

Today, the Acropolis is again a political symbol, important enough to Greeks that a project begins within the next few months to partially restore the half-destroyed Parthenon, piecing together many parts of the ruins to resemble some of the original grandeur.

Since 1974, when seven years of military dictatorship gave way to the restoration of democracy in Greece, the government has sought to restore and protect the monuments. In 1975, a Committee for the Preservation of the Acropolis Monuments was set up to deal with the damage caused by catastrophe, poorly conceived restoration, pollution, and just plain neglect.

The first damage fell 12 years after completion of the Parthenon, when an earthquake shook the area. Manolis Corres, the architect in charge of the current restoration, estimates the quake would have registered 8 on the Richter scale. The damage was immediately repaired.

There have been other tremors over the years. During a quake in 1981, a column in the southeastern corner of the Parthenon shifted more than an inch.

Although worrisome, earthquakes have not been the primary cause of damage to the Acropolis monuments. Mr. Corres says they caused ''not much damage. The construction was very rigid, not elastic. So there was very little resonance. Also, the columns are not attached but free-standing, so it could absorb very big shakes by hopping a little.''

Man must take the blame for the most serious damage. The first great destruction of the Parthenon was by fire sometime in the 3rd or 4th century AD. According to the most respected theories, the fire was set by northern invaders - either Herulians or Visigoths. The blaze cracked the marble inside the temple and destroyed all the wooden parts, including the ceiling and roof.

''The fire was definitely the most serious disaster to hit the Parthenon,'' asserted Corres, ''because it made the structure weaker, less able to resist later disasters.''

In the 6th century, Byzantine Emperor Justinian ordered the Parthenon transformed into the Cathedral of Athens dedicated to St. Sophia, and the inside of the temple was extensively remodeled. About the same time, the Erechtheum also became a church and the Propylaea the episcopal palace.

In 1205 the Franks took Athens, and the ruling de la Roche dukes moved into the Propylaea. They renamed the Parthenon Notre Dame of Athens. Later a 90-foot Frankish tower was erected on the south side of the Propylaea.

When Athens fell to the Ottoman Empire in 1458, the Parthenon became a mosque and a minaret was added. The aga - commander of the Acropolis - moved into the Propylaea and used part of it as a magazine. The Erechtheum became a harem for his wives.

Despite these changes, the buildings remained in good condition until the 17 th century, when lightning struck the magazine in the Propylaea, knocking down two columns and damaging the east portico.

In 1686 the Turks dismantled the small temple of Athena Nike - goddess of victory - on the south wing of the Propylaea to make room for a battery. And in 1687, the Venetians led by Morosini set up cannons on the Hill of the Muses and bombarded the Acropolis. A shell landed in the center of the Parthenon, which the Turks used as an ammunition store. The explosion knocked down 14 columns and destroyed the interior. Having captured the citadel, Morosini tried to remove some of the statuary with little success but with a great deal of harm, as several pieces were dropped and shattered.

From the time the Turks recaptured the Acropolis a year later until Greek independence in 1830, the monuments were surrounded by a maze of little houses - some of them built with fallen marble from the ancient monuments - inhabited mostly by Turks. In 1801, Thomas Bruce, Lord Elgin, the ambassador of Great Britain to the Porte, stripped the Parthenon of most of its frieze and removed many of its metopes and statues. During the process of removal, enormous injury was done to the building: Cornice blocks weighing 7 to 10 tons were thrown to the ground and broken.

Since Greek independence almost all the damage has come from ill-conceived restoration efforts and air pollution. In the mid-1830s, restorers destroyed all features that did not belong to the classical monument.

Several restoration programs followed in the next 100 years. They involved some return of the original components to their previous positions and efforts to strengthen the structures by inserting steel joints into the marble. But these components were inadequately protected from possible oxidation, and they later rusted and swelled, causing the marble to crack.

''This is the most urgent problem we face,'' said Dr. Theodore Skoulikidis, professor of physical chemistry at the National Technical University of Athens. ''We will remove them, except those whose removal would cause more damage than leaving them, and replace them with titanium, which is much stronger and more resistant.''

The ancient Greeks used iron joints, too, but they wrapped them in soft lead, thus protecting them and allowing them to expand and contract without hurting the marble. In addition, the ancient Greeks' work was so perfect that the buildings remained virtually airtight until catastrophes like the explosion of 1687.

''The work was so refined,'' according to Socrates Mavrommatis, official photographer of the Acropolis, ''that the places where two blocks joined were invisible. Originally the Parthenon looked like it was carved from a single piece of marble. In places, cracks in one block of marble have continued straight into an adjacent block as if they were a single unit, illustrating how precisely joined they were. This explains why no effort was made to ensure that the drums that formed each column were symmetrical with those on other columns: Only the final, whole shape was visible, not the individual components.

Although the ancient surfaces had eroded somewhat before, by far the greatest loss has occurred since 1956, when Athens' rapid industrialization began. Since then the city - which houses 40 percent of Greece's population - has surged up around the Acropolis and produced the nefos, the tawny cloud of sulfur dioxide, nitrogen oxide, carbon monoxide and dioxide, and other pollutants.

The nefosm has affected the monuments in various ways. Sulfur dioxide combines with rainwater to produce sulfuric acid, which turns the marble into gypsum. Wind and rain wash the gypsum off exposed surfaces. Particles of carbon and metal oxides that are suspended in the air are projected onto the monuments by the wind, producing a sand-blast effect that pocks the marble and encourages the gypsum to crumble. The pollution also accelerates the oxidation of the iron components of the monuments.

Since 1975 a team of architects, engineers, and archaeologists has conducted a painstaking study of the monuments, cataloging the thousands of fallen components. In addition, scientists led by Skoulikidis have carried out research to find a way to protect the surfaces.

In 1977 a plan to restore the Erechtheum was approved by an international conference of experts convened by the Greeks, who were concerned that the project receive international backing. ''This is not only a Greek treasure,'' asserted Corres, ''but one for the world.'' Since then the temple has been virtually completely dismantled and put back together again.

Iron parts have been replaced with titanium substitutes; fallen marble - when necessary fitted to new pieces of Pendelic marble from the ancient quarry outside Athens - has been returned to its original position; and the five caryatid maidens (a sixth was removed by Lord Elgin) have been placed in a nitrogen-filled case in the Acropolis museum and replaced by cement copies. Within the next few months the work will be complete and the scaffolding removed.

In mid-September a second international conference of experts convened in Athens to approve the initial phases of a plan for the Parthenon devised under the direction of Corres. The work, which will start in coming months, will cost approximately $5 million and is expected to take up to 10 years. Hundreds of fallen blocks will be lifted back into place, faulty work from previous restorations will be corrected, iron supports will be replaced.

Corres has designed a crane like the ancient one, and which will sit inside the temple and fold down out of sight.

Meanwhile masons - most of them from the island of Tinos, which possesses large marble quarries and has been a center for such work since the 16th century - will repair damaged blocks and fashion new parts where required for stability.

''We should not and will not reconstruct the temple,'' said architect Charalambos Bouras, a member of the Acropolis committee, ''but we will only put original stone back in place. This will make the temple more complete, but it will remain a ruin as it should.''

For the men working on the Parthenon, it is the summit of their craft. Each day, they say, they become more awed by the work of their ancient predecessors. ''They understand their role,'' said architect Charalambos Bouras, a member of the Acropolis committee, ''and thy perform it with great sensitivity.''

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