Italy has opened the hunt for Leonardo da Vinci's greatest lost fresco.
Evidence claiming the fabled fresco of the Battle of Anghiari was behind a secret wall in Florence's Palazzo Vecchio was first presented two years ago.
But experts weren't sure about it, and officials balked at the idea of knocking through a later wall painting by the famous art historian Giorgio Vasari in the Salone dei Cincequento.
Now it has been decided to build a scale model of the wall and recreate bits of the original fresco behind it - before coming back to hopefully find the real thing.
The whole process will take about a year.
The tests will take place at the University of San Diego, a major funder of the one-million-euro project along with three private foundations.
In order to put the fresco fragments together, experts will pore over well-known smaller copies as well as documents detailing Leonardo's painting technique.
They'll also compare the imagined work to existing Leonardo frescos like The Last Supper.
A barrage of equipment will be trained on the scale model. If they work they'll be used back in Florence.
One tool comes from the world of nuclear physics, explained art sleuth Maurizio Seracini - the only real-life character in Dan Brown's bestselling thriller The Da Vinci Code and the man who uncovered the wall in summer 2005.
The cutting-edge nuclear probe will carry out an analysis using "neutron activation," said Seracini, who has devoted 30 years of his life to finding the fresco.
"Point by point, we will get a map that will enable us to see what's behind Vasari's fresco," he said.
So-called 'georadar' technology created at the University of Florence will also be used.
Once the experts have a good idea where to look, they'll return to Florence to find what - if anything - is there.
"After all the polemics we're finally at the kick-off," said Culture Minister Francesco Rutelli.
"In a year's time we'll have a solution to the riddle".
Florence Mayor Leonardo Domenico said: "There is something underneath the Vasari, that's one thing we can be sure of".
"Now, with everyone working together, we'll find out what it is and what shape it's in".
Florence Provincial Chief Matteo Renzo said: "We're at a turning point. The Battle of Anghiari's mystery days are numbered".
And even if nothing turns up, Rutelli said, "we'll be able to use the technology for other treasure hunts".
The discovery of the wall in June 2005 raised new hopes of finding this Holy Grail of the art world.
But opinion has been divided.
Seracini believes the wall hides "significant" traces of the fresco.
The world's top Leonardo scholar, Carlo Pedretti, is convinced the fresco is hiding in the palazzo - the past and present hub of Florentine government.
"I really believe it's behind that wall," said Pedretti, director of the Armand Hammer Center for Leonardo Studies at the University of California in San Diego.
He said Renaissance accounts showed "the fresco can only be there".
Pedretti was also optimistic about finding the fresco virtually intact.
He rejected suggestions that Vasari might have damaged the fresco when he was told to cover it.
Vasari was asked to paint over masterworks by Giotto and Masaccio in two other Florentine sites, he noted, but left the underlying works intact.
"If he didn't damage those ones, why should he have done so with the Leonardo?"
This optimism contrasted with the initial views of then Florence art chief Antonio Paolucci who said "there's little or nothing behind that wall".
But Paolucci, a former culture minister, was later instrumental in setting up the international panel that has now decided what to do about the find.
'ANGHIARI' KNOWN FROM COPIES, SKETCHES.
Florence commissioned The Battle of Anghiari to celebrate its famous victory over Milan on June 29, 1440.
It was described by sculptor Benvenuto Cellini (1500-1571) as a "ground-breaking masterpiece" that any artist simply had to see and study.
In a 1549 letter to a Venetian friend, Florentine painter Anton Francesco Doni called it "a miraculous thing".
The work has long been known from sketches and copies.
But the original was thought lost for ever - a victim of Leonardo's typically unorthodox decision to jettison the traditional technique of applying paint to wet plaster.
Leonardo (1452-1519) needed time for his painstaking approach and so used oils directly on the dry plaster in Palazzo Vecchio, the symbol of Florentine civic pride.
Like the Last Supper in Milan it soon began to crumble, helped on its way by a thunderstorm that hit the unfinished building.
Leonardo gave up and headed for Milan.
Originally, the Leonardo work was to have stood opposite another grand martial fresco by Michelangelo, on the other side of the Salone.
But Michelangelo didn't even start it.
Experts have been sure, however, that some version of Anghiari survived. Under pressure from Seracini, officials knocked through two square metres of the Vasari in 1979 - but found nothing.
