One of the stranger supposed miracles in the Christian catalog is the miracle of incorruptibility. More common in Catholicism (I have yet to come across any Protestant examples), this term refers to a saint or holy person whose body miraculously refuses to decompose after death, instead remaining intact and even lifelike.
This apologist site lists some of the more prominent Catholic incorruptibles, though as is usual with apologetics, considerable exaggeration has crept in. For example, regarding one of the most famous incorruptibles, St. Bernadette Soubiros, the site reports how doctors were amazed at the supposedly flawless preservation of the body over fifty years after death. Yet when a reliquary was being prepared to display Bernadette's body (it is still on display today in a chapel in France), a decision was made to create wax masks to cover her face and hands. Visitors to Bernadette's shrine today are seeing wax, not skin. The Catholic site tactfully claims that this was done to cover up a "slight" skin discoloration on the face, but does not explain why a slight discoloration would require a full-face wax mask and wax to cover the hands as well. The Wikipedia article on Bernadette says, probably more accurately, that at that time the face had a blackish tinge and the eyes and nose were noticeably sunken.
Still, the surprising degree of preservation of this and other corpses does merit closer examination. What could account for a body remaining more or less intact for decades?
In some cases, the answer turns out to be obvious in retrospect. Consider the story of St. Margaret of Cortona, whose body has remained incorrupt and whole for over seven hundred years - seemingly a great miracle. Yet a recent forensic examination, commissioned by the Catholic church and described in the June 2001 edition of Discover, revealed the startling truth:
As Fulcheri gently lifted the hem of her dress up over her legs, all those assembled began to murmur. Several long incisions streaked along her thighs; other, deeper cuts ran along her abdomen and chest. Clearly made after death, they had been sewn shut with a whipstitch in coarse black thread. Saint Margaret had been artificially mummified.
The pathologists who examined Margaret's body later unearthed ecclesiastical records that told the whole story: the people of her town had asked the church to embalm her when she died. This had been done, with remarkable thoroughness. But the records of this fact had been lost, and over time, people forgot the circumstances of her preservation and simply began to assume that it was a miracle.
This story raises the question of how many other "incorruptible" saints might have undergone artificial preservation. Nevertheless, embalming does not seem to be the whole answer. The article notes that some other incorruptible bodies have been examined and showed no similar signs of human handiwork. Is this a miracle, or are there natural processes that can preserve a body?
If incorruptibility is a miracle, it is not solely a Christian one. Consider the case of Dashi-Dorzho Itigilov, a Russian Buddhist lama who died in 1927. Allegedly, he told his followers to exhume his body 30 years after his death, and when they did, they found it still intact, undecayed. The corpse was reburied, but 45 years later, in 2002, it was exhumed again and again discovered to be perfectly preserved. To this day, Itigilov's body is on display in the monastery in Ivolginsk, still sitting in the lotus position:
The lamas have dressed his body in a golden robe, with a blue sash laid across his lap. His eyes are closed, his features blurred, though the shape of his face and his nose certainly resemble the 1913 photograph. His hands remain flexible, his nails perfectly trimmed. His skin is leathery but soft. His head is still covered in short-trimmed hair.
(Other Buddhist monks, known as sokoshinbutsu, are also known to have well-preserved bodies, supposedly as the result of ritual ascetic practices they carried out prior to death.)
The fact that incorruptibility occurs in people other than Christian saints makes the miracle explanation considerably less likely. Far more likely is that natural processes, perhaps rare or only operating under the right circumstances, can conspire to preserve a body even in the absence of any special measures. As it happens, some candidate processes that fit the bill are known.
One example is called adipocere. In a damp, alkaline environment, fatty tissue in a body can undergo a chemical reaction that turns it into a hard, waxy, soaplike substance (which is why adipocere is often called "grave wax"). As Cecil Adams notes, the end result is a cadaver which looks "like something you'd find in a wax museum" ("albeit the George Romero wing", he adds dryly - which is fitting, considering that most claims of incorruptibility have more than a tinge of the macabre). Adipocere inhibits decomposition, preserves the shape of the body like a cast, and can last for centuries. It is very likely the explanation for many incorruptible corpses.
Another candidate is mummification. If left in a very cold, salty or dry environment, a body can become desiccated and resistant to decay. (Decay typically requires at least some moisture - this is why honey does not rot or ferment, because its low moisture content makes it impossible for bacteria to grow in it.) Natural mummies like Otzi the Iceman are well-known. Though these bodies may not be completely lifelike, they do remain intact, and with a little pious imagination (and perhaps some judicious use of wax), it's not hard to see how another claim of incorruptibility could be advanced.
Most important, and seemingly never considered by apologists, is this: How common is it for a body to remain intact? Many religious beliefs arise from the fallacy of counting the hits and forgetting the misses. Since most bodies are not exhumed, this is especially apt. It may be that postmortem preservation is common, but we do not know because we usually never check. In the absence of detailed evidence, the few undecayed bodies we know of should not be proclaimed to be extraordinary, and if these incorrupt Catholic saints are just a few among a much larger category of examples, the claim that their preservation is due to God's special favor would swiftly collapse.