4. It was made by some kind of energy release
According to an international team of scientists and other interested folk called the Yahoo Shroud Science Group, hypotheses about the genesis of the shroud "involving the Resurrection of Jesus of Nazareth cannot be rejected". Among them, the group members write, "are hypotheses correlated to an energy source coming from the enveloped or wrapped Man, [and] others correlated to surface electrostatic discharges caused by an electric field". Since these hypotheses appear to invoke processes unknown to science, which presumably occur during a return from the dead, it's technically true that science can't disprove them - nor really say anything about them at all.
Some, however, are not deterred by that. Italian chemist Giulio Fanti of the University of Padua has proposed that the image might have been burnt into the upper layers of the cloth by a burst of "radiant energy" - bright light, ultraviolet light, X-rays or streams of fundamental particles - emanating from the body itself. Fanti cites the account of Christ's Transfiguration, witnessed by Peter, John and James and recounted in Luke 9:29: "As he was praying, the appearance of his face changed, and his clothes became as bright as a flash of lightning." This is, to put it mildly, rather circumstantial evidence. But Fanti suggests we might at least test whether artificial sources of such radiation can produce a similar result on linen.
According to Raymond Rogers, all kinds of pseudoscientific theories have been put forward that invoke some mysterious radiation, which not only made the image itself but distorted the radiocarbon dating. In general they start from the notion that the shroud must be genuine and work backwards from that goal, he said. Little has changed in the decade and more since Rogers made this complaint. But still it has to be said that the piece of cloth Pope Francis will venerate is genuinely and stubbornly perplexing.