Human bodies are sometimes preserved by intentional human intervention, examples of this type of preservation would include the Egyptian mummies and the bodies of Lenin and Stalin.
Sometimes bodies are preserved by an accident of nature such as in the case of the 'bog bodies' found in various places in Europe. This process is called 'saponification' and the most notable thing about these bodies preserved by 'natural mummification' is that, once exposed to the air, the bodies decay and smell very unpleasant very quickly.
However sometimes bodies are found to be in a remarkable state of preservation for no apparent reason, not having been preserved as a result of human intervention nor by natural accident. These bodies are flexible and usually reported as exuding a very pleasant smell and also, in some cases, also a pleasantly smelling oil. The only common denominator for these inexplicable cases is the holiness of the person whose bodies have been preserved. One of the best examples of incorruptibility is that of Saint Bernadette of Lourdes who was buried normally in 1879 and disinterred 30 years later in order to move her remains, the body was found to be totally incorrupt and is exposed to this day at her convent in Nevers, France.
In spite of the enormous efforts and costs expended by scientists of the Soviet state to artificially preserve their leaders' bodies, their efforts were only partially successful and, of course, the techniques used were certainly not available centuries ago when many of the saints' bodies were discovered to be incorrupt.
There are about a hundred known cases among Catholic dead. In some cases the remains are on open display to this day, in others the remains are not on display but we have photographs of the remains before they were re-interred, but in all cases we do have certification by several competent witnesses.
To see the photographs of some of the bodies and to read about the lives of those involved go to INCORRUPT SAINTS