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Read more: “Death: A special report on the inevitable“

IT’S NOT a nice thing to contemplate. But set aside the thought of any of the below befalling you or your loved ones, and what happens to our mortal remains when we are no longer using them is pretty fascinating. If nothing else, it proves that nature is ruthlessly efficient at clearing up its messes.

At least it can be. Very few people in the modern world get to be dead the old-fashioned way – out in the open, exposed to the elements. Of those that do, the speed at which the body turns to dust depends on a mix of factors including temperature, moisture and the animals, insects and microbes that happen to be there. In a relatively warm and moist spot with plenty of insects and scavengers, a human body can be turned to bones within a few weeks and disappear completely in months.

“Very few people in the modern world get to be dead in the old fashioned way”

But what about the majority of bodies, which get refrigerated soon after death, then embalmed and put in a coffin? Again, it depends. Temperature and moisture are still the most important factors, but numerous others play a part, from how well the body was embalmed to the tightness of the seal on the coffin, the acidity of the soil and that of the groundwater which will eventually seep inside. All of this means that it is impossible to predict how long a particular body’s final journey might take – it can be anything from months to decades.

What we can say, though, is that whatever the timescale, the vast majority of bodies will go through the same stages of decomposition.

First comes the “fresh” stage. Within minutes of death, carbon dioxide starts to accumulate in the blood, making it more acidic. This causes cells to burst open and spill enzymes which start to digest tissues from within.

The first visible sign of decomposition comes after half an hour or so, as blood pools in the parts of the body closest to the ground. At first this looks like purplish-red blotches; over the next day or so it turns into an almost continuous purplish mark known as livor mortis. The rest of the body turns deathly pale.

Around the same time, muscles go floppy and then stiffen as rigor mortis sets in. In life, pumps in the membranes of muscle cells control the amount of calcium ions in the cell – high levels stimulate contraction and low levels allow relaxation. The pumps no longer work after death, so calcium ions diffuse into the cells from the higher concentration outside, causing the muscles to contract.

Rigor mortis passes after two to three days. But what looks like relaxation is actually rot setting in, as enzymes break down the proteins that held the muscles in their contracted state.

Embalming the body stops the rot in its tracks, at least temporarily. Unlike ancient Egyptian embalmers, who aimed to keep the body intact for all eternity, modern embalming is designed to make a corpse look presentable and keep it in one piece long enough to organise a funeral.

This is done by disinfecting the body and replacing the blood and other fluids with a mixture of water, dye and preservatives, usually including formaldehyde. The dye is to restore something resembling a healthy skin tone, while the formaldehyde preserves the body in several ways, first by repelling insects and killing bacteria. It also inactivates the body’s enzymes and makes the tissues more resistant to decomposition by adding cross links to the chains of amino acids that make up proteins.

This protective effect only lasts so long, though, leaving the body more or less back where it started.

The next stage, putrefaction, gets a little ugly – not to mention smelly – as the enzymes, aided and abetted by microbes, get to work. After 48 hours or so, when enough nutrient-rich fluid has spilled from the burst cells, these microbes spread rapidly. The main beneficiaries are among the 100 trillion bacteria that have spent their lives living in harmony with us in our guts. As they break down proteins they churn out two compounds with names as stinky as their smells, putrescine and cadaverine, and these give a corpse its repulsive odour.

From the outside, putrefaction can be seen as a green hue, slowly spreading from the front of the belly across the chest and down the body. The green colour comes from the action of anaerobic bacteria, which convert haemoglobin in the blood to sulfhaemoglobin.

All this bacterial action also creates gases, including hydrogen, carbon dioxide, methane, ammonia, sulphur dioxide and hydrogen sulphide. These contribute to the stink and also distort the body, blowing it up like a balloon and eventually, after a month or so, bursting it open. Hydrogen sulphide also combines with the iron in haemoglobin to make the black-coloured iron sulphide, which turns the skin darker.

This heralds the start of the third stage: active decay. The rate of decomposition now speeds up and what is left of the flesh is rapidly consumed, until all that remains is the skeleton. Sometimes, something else can happen too, though. If the body happens to be in particularly cold soil, a waxy covering called adipocere, or grave wax, might form. Adipocere is a particularly spooky side effect of the work of some anaerobic bacteria, such as Clostridium perfringens, as they digest body fat. It takes around a month to start forming and can leave the corpse with what looks like a wax coating.

The final stage – breakdown of the skeleton – takes the longest. For the bones to disappear the hard mineral parts need to be broken down. This happens if they come into contact with acidic soil or water, and speeds up if they are mechanically broken up by tree roots or animals. Once the hard stuff is gone, the body’s last proteins, including the collagen that once gave the bones flexibility, succumb to bacteria and fungi and disappear.

There are some cases where this sequence of events doesn’t play out at all and the body doesn’t get a chance to decay. If the corpse is kept completely dry bacteria can’t do their work and the tissues will mummify. The same goes for bodies that fall into natural preservatives such as bogs, salt marshes or snow, where bacteria don’t thrive and the body’s enzymes don’t work.

Then there are the rare cases when a person dies in the company of hungry scavengers. In these cases the body can be stripped to the bones and chewed into tiny pieces in a matter of days. The same can happen under the sea.

“In rare cases a body can be stripped to the bone and chewed into tiny pieces in days”

Of course, without a bog, dog, shark or icy grave to hand, the only way to avoid the harsh realities of decay is cremation. In a chamber heated to 750 °C the coffin and entire corpse can be burned in under 3 hours. After that, the ashes are passed through a grinder called a cremulator to take care of any particularly big or stubborn bones that haven’t completely burned and turn the entire remains into fine ash.

And that, as they say, is that. It may not be pretty but it’s one of the few definites in life: ashes to ashes, dust to dust, in the end there’s not a lot left.