Take for example the Apparition of our Lady at Guadalupe:
...physicist Aldofo Orozco told participants at the International Marian Congress on Our Lady of Guadalupe that there is no scientific explanation for the 478 years of preservation of the tilma on which the image appears.
According to Orozco, all cloths similar to the tilma that have been placed in the salty and humid environment around the basilica have lasted no longer than 10 years. In contrast, Orozco said, “The original Tilma was exposed for approximately 116 years without any kind of protection, receiving all the infrared and ultraviolet radiation from the tens of thousands of candles near it and exposed to the humid and salty air around the temple.”
The fabric, according to Orozco, is made from fibers of the agave plant. The back side of the fabric is rough and coarse, while the front side is as soft as silk.
He also mentioned two of the miracles associated with the tilma.
The first occurred in 1785, when a worker accidentally spilled a 50% nitric acid solvent on the right side of the cloth. “Besides any natural explanation, the acid has not destroyed the fabric of the cloth; indeed it has not even destroyed the colored parts of the image,” Orozco said.
The second miracle was the explosion of a bomb near the tilma in 1921. Orozco recalled that the explosion broke the marble floor and widows 150 meters from the explosion, but “unexpectedly, neither the tilma nor the normal glass that protected the tilma was damaged or broken.” The only damage near it was a brass crucifix that was twisted by the blast...
(Source: National Catholic Register)