The Shroud of Turin

[Stranger in a strange land]

The most likely answer is that it's a hoax from Late Antiquity or the Middle Ages.

The shroud is old and from the Near East. Prior to the appearance of the Shroud of Turin, there was a similar relic called the Image of Edessa on display in Constantinople. After Constantinople was sacked by the Crusaders in 1204, the Image of Edessa disappeared from the historical record, and it's thought by some that it later reappeared as the Shroud of Turin. However, recent radiocarbon dating casts doubt upon this hypothesis, and suggests that the shroud was produced in the late 13th or early 14th centuries. Defenders of the Edessa Image hypothesis counter that bacteria-associated residue in the shroud would make it seem younger than it actually is.

One possibility the reason it gives off such an eerie photographic negative is that it was created using a primitive photographic technique. It was once suggested that Leonardo Da Vinci may have created the shroud using a crude camera, though this is almost certainly false, as there are references to the Shroud in writings from before Leonardo's lifetime.