How'd the Shroud of Turin Get It's 'Jesus Trace'?

Sunday will mark a special day in Christendom.
On the Holy day this week, Pope Francis will venerate a piece from the Shroud of Turin.
The Archbishop of Turin confirmed that the Pope will recognize the mysterious linen cloth millions of Christians believe Jesus Christ died in.
However the veneration does not mean the Catholic Church accepts the item's authenticity; they have always left that explanation to science.
Unfortunately the curious minds that have tested the shroud have done no better to determine whether Christ died in the garment or not.
Theories range from a natural chemical process of sugary carbohydrates changing color, to an early example of a photographic negative.