eli5: Why do drowned bodies sink and then float?


eli5: Why do drowned bodies sink and then float?

In: Biology



Not everyone’s the same, but most people are just about buoyant when they’re breathing, so when they stop, all those sinuses and air sacs fill with water. Now that they’re a little denser than their surroundings, the lighter (per unit volume) water pushes them down. But, the water wasn’t counting on all the bacteria that was eating that body and can now go ham since the defenses have turned off with the death of the brain. Those bacteria eventually convert enough of their food into gases as they digest and those gases combine into pockets that are less dense than the surrounding water, so the body gets pushed back up.

As someone who has recovered bodies from waterways… The bodies are almost neutral bouyancy at time of death, but then take water in and loose air from those balloons we call lungs after death causing the remains to sink.

Go float in a pool, and exhale. See how fast you sink without that air in your lungs.

The resurfacing occurs due to mostly natural bacterial flora inside your body beginning the decomposition process. This produces gasses inside the carcass, causing swelling and bloating. This eventually acts just like the air in your lungs and the body resurfaces. Actually much more boyant.

They sink because the air in their lungs gets expelled and they become heavier than water.
They then start to decompose and when there is enough of a gas buildup, they float


Basically a body turns into a flesh balloon as the bacteria in its guts consume its internal organs and fart gases that are lighter than air which inflated the corpse and makes it float or at least until a the gas escapes.

First let’s talk different components of our bodies and their density relative to water.

Simplifying alot but

We have lots of water – obviosuly same density as water.

Proteins/muscle: slightly more dense than water so will sink.

Fat: less dense than water so floats.

Bone – much more dense than water and also sinks.

Finally we have some amount of air within lungs – much less dense than water .

An average weight person is typically on average (average density of the entire body) slightly less dense than water and so they float*. When a person drowns the air within the lungs is replaced with water which changes the overall density to be greater than water and so they sink. Over time decomposition of the body leads to gas accumulation within the body which changes the overall density back to being less than water and so they so float back to the top.

*Used to be a lifeguard and part of the certification was to tread water and support one of the other people in the class for like a minute. At the time I was like 130 lbs and average build – so slightly less dense than water and floated. One of my classmates was probably 200-240 lbs of pure muscle and sank like a rock and another was probably 300 lbs of mostly fat and floated easily. Doing the cerification with the super buff dude was 1000x harder the fatter guy.

when a body is decomposing the bacteria in the gut go crazy, since theres no immune system to keep them in check. while theyre feeding and multiplying the of course have to get rid of waste products, which is, more often than not, gasses. The gasses are lighter than water, and they turn the body into a sort of beach ball, so it floats because of all the gasses.

Well the density of the human body is slightly higher than water, but when you breathe in your volume increases and than you are less dense than water, so you float. When you die many things happen and gasses are released. This reduces density. So you become less dense than water and float. And density is mass over volume. The reason a giant boat can float but the steel sinks.


Well, human bodies are denser than water. However, our guts are filled with gasses, that give us close to neutral buoyancy. That is, why if you deeply exhale in a pool, you sink. If you die, water replaces the gas in your lungs and you sink. As you rot, your body is broken down and decomposition gases like methane begin building up. They have higher buoyancy than water, and so the body rises to the surface.

when you drown your lungs fill with fluid and you sink. once dead your body begins to break down and the bacteria in your intestines start eating you. as they eat the fart and fill your body with gasses, which make you buoyant and you then float back up to the surface.

Not knowing this answer = you’re clearly not a serial killer.

Don’t worry, folks. OP didn’t do it.