How did Ötzi's body survive five millennia in the Alpine ice?

• In this exclusive Horizon website presentation, Thomas Bereuter from the Vienna University of Technology explains what turned a corpse into an ice mummy.

One of the questions which has intrigued scientists is how Ötzi came to be so well preserved for over five thousand years. Thomas Bereuter has studied the chemical processes that take place in bodies after death. When he heard about the Iceman, he was keen to investigate the case. One change at least was obvious...

Bereuter wanted to know if drying out was the only change that had occurred to the Iceman, so he decided to examine the body. He could see that iceman's fingernails and toenails had dropped off. There was no body hair. And also the outermost layer of the skin, the epidermis, was missing. A purely dried out body should have retained all these parts. So something else had occurred during those five thousand years...

To find out what else had happened to the body, Bereuter obtained a sample of the iceman's tissue and analysed it for its chemical contents. He was looking for any chemical changes that had occurred, in particular to the body fat. What he found was very surprising...

Bereuter had found that the layers of fat under the skin of the corpse had transformed into adipocere, also known as grave wax. Although it's derived from fat, it looks nothing like it...

Adipocere is almost totally resistant to decay, and so the combination of freezing, drying and conversion to adipocere explains why the Iceman's body is so well preserved.

The discovery of adipocere is important for another reason because it can only form in certain conditions - when a body is in liquid water. So this evidence supports the theory that Ötzi had not always been frozen in ice, at some point the ice melted. Dr Bereuter believes that this may have occurred in Roman times, around 100 AD, when climate records show that the area was warmer. At this time, the glacier would have receded leaving Ötzi floating in a pool of melt-water. The adipocere could have formed in these conditions and would have helped prevent the body decaying in the water.

For a closer look at Ötzi's clothing and artefacts, view the Iceman gallery.