Computer image processing has discovered the ghostly image of a man on the Turin Shroud, but on its reverse side, rather than on the top of the famous relic.
The image matches that of the face on the front side of the shroud, with faint details of a nose, eyes, hair, beard and moustache.
The discovery is published today in a scientific paper by two Italian scientists. The shroud is among the most controversial Christian relics. Supporters say the handwoven cloth is the burial shroud of Jesus, and that the "image" visible on its front - the smudged outline of a man's body - is proof of the Resurrection.
But scientists who have studied the image have repeatedly said it is a convincing and clever fake, probably from medieval times, "no miracles are necessary to explain the [front] image" and it "can be explained by reference to highly probable, well-known chemical reactions".
But that assertion will meet a renewed challenge with the new research, in the peer-reviewed Journal of Optics A: Pure and Applied Optics. The "underside" of the shroud has rarely been seen. It is hidden under a piece of cloth that was sewn on by nuns in 1534 after a fire that blackened parts of the huge cloth, which measures 14ft3in by 3ft7in (4m by 1m).
The reverse side of the shroud was scrutinised in detail for the first time only in 2002, when the extra cloth was unstitched during restoration. The latest research builds on photographs taken then which were published in a book by one of the church's keepers of the shroud.
"I was caught by the perception of a faint image on the back surface of the shroud," says Giulio Fanti, professor of mechanical and thermic measurements at Padua University, who is one of the authors of the latest paper. "I thought that perhaps there was much more that wasn't visible to the naked eye," he said.
Though nothing stood out to the naked eye, Professor Fanti and a colleague, Roberto Manniolo, used sophisticated image-processing techniques that can extract pictures from background "noise".
Using them, they discovered the image of a face on the back side of the shroud, lying directly behind the familiar front image, which seems to show a bearded man with the marks of crucifixion.
"Though the image is very faint, features such as nose, eyes, hair, beard and moustaches are clearly visible," Professor Fanto says. "There are some slight differences with the known face. For example, the nose on the reverse side shows the same extension of both nostrils, unlike the front side, in which the right nostril is less evident."
But the enhancement procedure did not uncover an image of the body to match that on the front. "If it does exist, it is masked by the noise of the digital image itself," the scientist adds. "But we found what is probably the image of the hands."
The scientific paper is careful not to make any claims about how the image may have got there. "We were very insistent that there should be no religious speculation," a spokesman for the Institute of Physics says. "It is a paper that describes what they did and found."
The discovery of the second face may seem to strengthen claims that the shroud is in fact a fake: any paint or liquid used by forgers to make a print on the cloth would have soaked through. But Professor Fanti noted that on both sides the facial image is "superficial", involving only the outermost linen fibres.
"When a cross-section of the fabric is made, one extremely superficial image appears above and one below, but there is nothing in the middle. It is extremely difficult to make a fake with these features."
He says this "double superficiality" could answer how the image got on to the cloth. In the paper, he mentions a corona discharge, which can occur if the corpse is in an electric field and the atmosphere is ionised. That could happen for a body stored in a cave with quartz rocks prone to earthquakes, because that could release the radioactive gas radon.
In 1988, the Vatican approved carbon-dating tests. Three reputable laboratories in Oxford, Zurich and Tucson, Arizona, concluded that the Shroud was made between 1260 to 1390.
Ray Rogers, one of the scientists, has said: "The characteristics of the image can be explained by reference to highly probable, well-known chemical reactions. No miracles are necessary to explain the image."