Question: How do we understand unrotten dead bodies of disbelievers? We know that bodies of Prophets and those close to Allah do not rot; however, there are cases of disbelievers, who are no saints, whose bodies have not decomposed. How do we understand this phenomenon?
Answer: The Islamic perspective has been covered in the link in footnote 1 and so these shall not be discussed. We shall discuss the phenomenon of bodies of disbelievers not decomposing.
Unrotten vs. undecayed bodies
There is a difference between a body not rotting and a body not decaying. What we see in the link in footnote 2 are cases of unrotten bodies while they have suffered heavy decay. They have not been eaten by insects which is why they have not rotten; however, they have suffered decay and their current look is not the same as what it would have been at the time of their deaths.
Punishment of the grave
The following statements of the Prophet (ﷺ) are interesting and important:
إِنَّهُمْ يُعَذَّبُونَ عَذَابًا تَسْمَعُهُ الْبَهَائِمُ كُلُّهَا
They are tormented in a manner that all the animals can hear [Sunan an-Nasa’i 2067].
فَيَضْرِبُهُ بِمِطْرَاقٍ مِنْ حَدِيدٍ بَيْنَ أُذُنَيْهِ فَيَصِيحُ صَيْحَةً يَسْمَعُهَا الْخَلْقُ غَيْرَ الثَّقَلَيْنِ
He will then give him a blow between his ears with an iron hammer and will utter a shout which will be heard by all the creatures (near him) with the exception of men and jinn [Sunan Abi Dawud 4751].
Some of the dead are punished in the grave in such a way that their screams are heard by the creatures except the humans and the jinn. Some or all of these bodies could have been punished so severely and their screams heard by all except mankind and jinn that the insects got terrified and refused to eat them.
All disbelieving bodies should remain unrotten then?
One may argue that every disbeliever’s dead body should remain unrotten as they are punished in the grave and hence insects should not eat them. The answer is that these are matters of the unseen; we don’t know exactly how they are beaten, whether each and every disbeliever is beaten or only the worst and cruel of them, whether all of them shout or only some of them and so on. Delving into this would only be guesswork and so we leave it there.
Unrotten Muslim dead bodies
The unrotten and undecayed Muslim dead bodies that are discovered also have extremely pleasant smell with them and hence, they are not the same as these disbelieving dead bodies. We have seen above that there is a difference between rotting and decaying.
It is important to note that every Prophet or people close to Allah do not rot but not every unrotten one is a Messenger or close to Allah.
Those close to Allah do not rot neither do they decay; however, what we have witnessed of some of these disbelievers is that the insects and the earth did not eat them but their bodies did not remain preserved as original. They decayed.
Shaykh Muhammad Saalih al-Munajjid states:
It may be due to natural factors in the body itself, or in the soil (in which he is buried) or in that location. This is important research that scientists and experts could investigate and find out the scientific causes, based on a study of the composition of the soil and the types of bacteria present in it, or finding out whether some substances were applied to the body (that led to its preservation). All of these are possible causes of its being preserved. This is important research that should be carried out by scientists and researchers.
It may also be due to deceptive actions on the part of some fanatics who show the body as if it is preserved from disintegration, when in fact it is decaying and decomposing, but it appears to be intact because of clothing, filling in and application of wax to make the body appear to be intact, so as to convince people of the soundness of a particular religion or sect that the deceased used to follow. Focusing on emotional preaching and deceit, far removed from research based on reason, is a well-known tactic among many of the followers of various religions and sects, but at the end of the day it backfires and they cannot deceive people or their followers forever.
The body may indeed be intact, but there is a difference between this appearance of being intact and the body that appears fresh, as if the person only recently died. The former happens in many cases, in which the body appears as though no part of it is missing, but a single touch is sufficient to make it crumble into dust. As for cases in which the body appears fresh, and in some instances a pleasant fragrance comes from the body and sometimes blood is even seen to gush forth from it, such cases are rare, and that is what we hope may be a sign of the person’s righteousness and his honour before Allah, may He be glorified and exalted, if the person was a Muslim and righteous. But still we cannot be certain of that, and we have no shar‘i evidence to prove it. Rather it is no more than the hope that it is a sign of Allah honouring such people, as explained above.
Conclusion: we call people to research and study on the basis of reason the phenomenon of incorruptibility of some bodies in their graves, and to study the matter from all angles, both biological and theological. We have not seen in any Islamic textbook anyone who regards such phenomena as a sign that the deceased was following the right sect or religion. Similarly, we have not come across anyone among Muslim scholars who quotes that as evidence in his debates with non-Muslims, because in fact it is not evidence.
This is contrary to what the clergy want, concerning whom there is research and specialist websites, who use wax and preservatives in order to show the bodies of their leaders and saints as if they have been preserved by the power of God, and as if this is a divine message calling people to believe in Christianity or Judaism. All of that comes under the heading of exaggeration and using emotion and deception, but the church cannot fool people who possess reason.
Indeed, Allah knows the best.
References and footnotes:
 SPECIAL BODIES WHICH DON’T DECOMPOSE
 5 Bodies That Refused to Rot
 Technically there may not be much difference between rotting and decaying but for the sake of clarification, I have defined them differently above.