The infamous case of Melissa Caddick highlighted the complexity of marine forensics, and how hard it is to piece together the sequence of events that led to human remains winding up in water. The grim discovery of a foot in a shoe indicated her body may have been submerged at depth before it broke loose and floated to the surface.
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So why do some human remains stay in the depths while others wash up on beaches? And do dead bodies sink or float? Let's take a look at what the science has to say.
Do dead bodies sink?
Human beings are very wet. It’s true. From our eyes to our muscles and bones (side note: why do we say bone dry?) we’re saturated with all kinds of fluids, and that has a significant influence on what our dead bodies do if a corpse is left in water.
Being so wet gives us about the same density as water, which is why we have to swim to stay afloat and can only do so if our lungs are filled with air. In the same way that someone in a pool can sink to the bottom by blowing the air out of their lungs, a dead body will sink without its internal buoyancy aids.
However, when we die the lungs aren’t necessarily empty straight away, and how we died can greatly alter this. People who have drowned, for instance, will sink much faster compared to somebody who has died a different way.
Furthermore, dead bodies may behave differently in fresh vs salt water. A 1977 paper demonstrated this in establishing the gravity and buoyancy of 98 men based on lung volume, concluding that 69 percent of the men would float in seawater with the lung volume of a recently dead person, while only 7 percent would float in freshwater.
How quickly do dead bodies sink?
A 2017 study found that the time it takes for a drowning victim whose lungs have filled with water to sink is very short. Different combinations of variables showed times of less than 7 seconds for sinking, with the longest time being around 10 seconds. The longer sinking times seemed to be linked to bodies with extremely small chest sizes.
Water depth, current flow, body type, and water density could also influence the sinking time. Understanding how quickly bodies sink is important because it can significantly alter the approach needed when searching for a missing person, as a longer sinking time can lead to greater movement of the body, making them harder to find.
Why do some dead bodies float?
Dead bodies washing ashore or being sighted floating at sea aren’t uncommon, so why is it that these bodies have become buoyant after death? It all comes down to putrefaction and the changes we see in a corpse as the tissues start to decompose, producing gases.
“[Death] is a failure of our biological systems that prevents a return to chemical equilibrium, and everything that then proceeds from that point onwards is typified to an extent by return to equilibrium with the exception where you've got all these biotic agents acting on the body,” said forensic expert Dr Devin Finaughty to IFLScience. “That’s because once all of that stops working, your immune system stops working, and because your immune system stops working, the bacteria that are intrinsic to our guts, which outnumber our cells 20-to-one, they are no longer constrained. So, they have a massive food bonanza and will start to consume everything.”
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“They'll go through the lining of your gastrointestinal system and into your bloodstream and start to spread out throughout the body. They will start to metabolise substrates in your body, starting off with the sugars that are on the surface of our cells and that are being leached out of the cells. Then they'll proceed towards more complex carbohydrates and fats. That is truly the start of decomposition and their metabolism produces a range of by-products called volatile organic compounds, which is what we smell as death.”
Some of these gases escape the body, which is why the smell of a corpse can be very noticeable, but some of those gases will – for a time – be trapped inside. This acts like a form of buoyancy replacing the now-empty lungs, filling the body with air in a way that is sometimes significant enough to make it rise to the surface. The time it takes for this to happen varies, and not all bodies will resurface, but it’s typically a matter of days if the body isn’t tethered to anything.
Everything from currents to salinity and marine life can alter the speed of decomposition, but one of the biggest factors is temperature. Decomposition happens more slowly in water than in air, more slowly in salt water than freshwater, and even slower still in water that is very cold. For this reason, some bodies in the coldest depths may never resurface, and often aren’t found until they’re skeletons.
What happens to bodies that float?
Even bodies attached to weights have been able to resurface, and when this happens, it changes the game in terms of what organisms within the decomposition ecosystem can access the bodies. Exposed sections of the body can become food for scavengers, mold, and algae, and will sometimes mummify.
All of this information can be used in marine forensics to try and ascertain how long the body has been in the water, how long it has been at the surface, and the window of time in which it could have been moving from where it entered the water. As with all forensic science, it isn’t easy, but each breakthrough in our understanding of what happens to dead bodies in water gets us closer to being able to find those lost at sea.
