Just about everything bad that can happen to a painting has happened to brothers Hubert and Jan van Eyck's , also known as the Ghent Altarpiece.

It's almost been destroyed in a fire, was nearly burned by rioting Calvinists, it's been forged, pillaged, dismembered, censored, stolen by Napoleon, hunted in the first world war, sold by a renegade cleric, then stolen repeatedly during the second world war, before being rescued by The Monuments Men, miners and a team of commando double-agents.

The fact that it was the work the Nazis were most desperate to steal - Hermann Goring wanted it for his private collection, Adolf Hitler as the centrepiece of his super-museum - only increased its renown.

It's easy to argue the artwork is the most influential painting ever made: it's the world's first major oil painting, and is laced with Catholic mysticism. It's almost an A to Z of Christianity - from the annunciation to the symbolic sacrifice of Christ, with the "mystic lamb" on an altar, bleeding into the holy grail.

On April 11, 1934, one of its 12 panels - depicting The Just (or Righteous) Judges - was stolen from the St Bavo Cathedral in a heist that has never been solved, although the case is still open and new leads are followed regularly.

The theft was followed by a ransom demand for one million Belgian francs. As a show of good faith, the ransomer returned one of the panel's two parts (a grisaille painting of St John the Baptist). But police remained baffled.

Then a stockbroker, Arsene Goedertier, had a heart attack at a Catholic political rally. His lawyer, Georges de Vos, claimed that just before he died, Goedertier whispered: "I alone know where the is. The information is in the drawer on the right of my writing table."

There, the lawyer found carbon copies of the ransom notes, plus an unsent note with a tantalising clue about the stolen panel's whereabouts: "[It] rests in a place where neither I, nor anybody else, can take it away without arousing the attention of the public."

But if Goedertier did steal the panel, why? The church was defensive, and there was an air of cover-up - plus evidence members of the diocese were involved. One theory goes that a group of church members, Goedertier among them, were involved in an investment scheme that lost church money. Rather than admit their failure, they stole the panel and ransomed it to cover the losses. But Goedertier was wealthy and devout; it seemed odd he would extort his beloved diocese.

One man who has done more than anyone to shed light on the case is Karel Mortier. In 1963, the former chief of the Ghent police flew to Germany to track down the widow of Heinrich Kohn, an art detective assigned to look for the lost Judges panel a decade after the theft. Kohn's 176-page file on his search showed he was convinced the panel was hidden inside the cathedral and that it was in the possession of the then-canon, Gabriel van den Gheyn (who had also kept the painting safely hidden during the first world war). The question is why he did not reveal the panel's whereabouts after the war.

Mortier, now in his 80s, still receives new clues: he estimates he has been contacted about 350 possible locations for the panel. St Bavo has been searched six times since the second world war; Mortier even supervised an X-ray of the cathedral to a depth of 10 metres.

The case remains alive in the public's imagination too: in 1995, Goedertier's skull was illegally excavated by an amateur detective. Someone then stole the skull to host a seance and ask Goedertier about the theft. In 2004, Mortier ran DNA tests on the stamps of the ransom notes searching for the thief's saliva. The tests were inconclusive.

The detective now in charge of the case, Jan de Kesel, made a major discovery when he tracked a lead to a church in Goedertier's hometown, and found the outline of a panel with the exact dimensions of the lost Judges. Was this where Van Den Gheyn hid it from the Nazis?

But there are holes in every theory. The attorney general's office has a 2,000-page file on the theft and the Canon of St Bavo still gets tip-offs that don't amount to anything - except that this theft is still aching to be solved.

This article appeared in the South China Morning Post print edition as: Mystery of stolen panel still baffles, 80 years later

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