The Greek government launched a project to restore the Parthenon and other buildings on the world heritage site in 1975, but it was not until 1983 that work started.
Scaffolding has been up somewhere around the ancient temple ever since. But from now until September, the exterior of the Parthenon will be scaffold-free.
Building the Parthenon took nine years from 447 BC and the sculptural decorations took another 10 years to complete. Restoration has already taken longer than it took to build.
"We treat every piece of marble like a piece of art so we have to respect it," Mary Ioannidou, the head of restoration told Reuters during a tour of the temple.
"The ancient Greeks had the possibility that if a block failed, to leave it and take another one, but we can't do it so we have to treat it with great respect."
Over the years, the Parthenon has suffered from fire, war, revolution, looting, misguided restoration and pollution.
It became a church for nearly 1,000 years and served as a mosque under the Ottomans for nearly 400 years after that.
The greatest blow to the structure though came in 1687 when a Venetian mortar ignited the Ottoman Turkish gunpowder store inside and widespread looting followed. British Ambassador Lord Elgin then removed large chunks of the sculptures from 1801.
Between 1898 and 1938, restoration workers rebuilt large parts of the building and concreted in parts of the columns and blocks that were missing. But they used iron ties to hold the blocks together and replaced many in the wrong place.
The iron ties have since rusted and as they did so expanded causing cracks to appear. The ancients also used iron ties, but coated them in lead to prevent rust. They have lasted well.
The team of archaeologists, marble cutters, architects, and civil and chemical engineers, dismantled 1,852 metric tons of marble and began the painstaking task of attempting to put it back again in the right place, adding other fragments they found.
"It's like a huge puzzle," said Ioannidou with a wry smile.
Titanium is now used to tie the blocks and columns together which is highly resistant to corrosion.
New marble has been crafted to fill in some of the gaps left by the concrete and allow blocks of the original marble to be returned to their place on the Parthenon's stonework.
The original quarry for the marble on Mount Penteli is now itself a protected historical site, but marble has been cut from the other side of the same mountain.
"It's almost the same but not exactly the same," said Ioannidou. The new marble stands out in a much lighter color than the original.
"One of the principles of our restoration is not to cheat the visitor. Everyone can understand the parts that are ancient and those that are original," said Ioannidou.
As for the color, that will fade. "If you come here in 10 years the color will be almost the same," she said.
In September though, the scaffolding will be up again on the western facade and that project will last at least another three years. Efforts to piece together the walls of the inner chamber of the temple are already underway.
For some, restoring the Parthenon is their life's work. Marble-cutter Ignatius Hiou has worked there for 18 years.
"If I could do this until the day I die, I will be happy," he said.
(Additional reporting by Deborah Kyvrikosaios; Editing by Paul Casciato)
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