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It’s almost Halloween, which means it’s time for the traditional binging on candy and horror movies. And while you’re watching Night of the Living Dead for the bazillionth time, you might start to ponder what you’d do to survive a zombie apocalypse.

Your survival hinges upon picking the right place to hunker down and to figure that out, it’s essential to know whether zombies float or sink. After all, if they sink, then your best bet for shelter may be a large, well-stocked yacht just out of their reach. Unfortunately, scientists haven’t found any zombies to experiment on… yet.

But they do know a lot about the properties of bodies after death, thanks to forensic studies and work conducted on our animal body doubles, pigs. And the short answer is: it depends. Zombies can probably sink or float, depending on factors like age and water temperature.

If you’ve ever been swimming, you know that humans are pretty close to the same density as water. So, generally, if you take a deep breath, you become slightly less dense than water, so you can float. If you empty your lungs, you become slightly denser than the water, so you sink.

Most zombie movies seem to agree that zombies don’t breathe. So unless air is trapped in their lungs, which can happen, a new zombie would sink to the bottom of a body of water. But it won’t stay on the bottom for very long.

That’s because the human gut is full of microbes, and when a person dies, not all of these microbes die with them. Some begin to digest the body from the inside out instead. And as they break down and consume tissues, they produce gases like hydrogen sulfide, methane, cadaverine, and putrescine.

These gases usually become trapped in the gut, eventually causing the body to float. How long that takes can vary, but in a 2004 experiment, scientists put dead pigs in the ocean for observation, hopefully in a secluded area, and found that the bodies started to float again after about 3 days and remained floating for several weeks. Since zombies rot on the outside, we’re assuming that they aren’t somehow immune to this type of internal rotting either.

So right when the zombie turns, they’d probably sink, but within a week, they’d float, and then they’d continue floating until the gases are somehow released. It’s possible that their body movements like frantically trying to dog-paddle after you to get at your brains would help the trapped gases work their way out, resulting in presumably horrific zombie flatulence and also the zombie sinking again. Temperature can also affect how quickly these bacteria work and how quickly gases build up.

Just like a refrigerator, a cold ocean or lake could slow down bacterial growth, so it’d take longer before the zombie starts to float. There are just a lot of variables to consider, so you can’t count on zombies sinking or floating when building your apocalypse safehouse. But… they might be less coordinated in the water, and draw unwelcome attention from hungry fish….

So maybe a yacht isn’t such a bad an idea after all. Perhaps a more important question is: can fish become zombies? Thanks to Chantelle for asking, and as always, thanks to all of you who support SciShow on Patreon.

If you want to pose questions like this or just get some really neat stuff you can’t get anywhere else, you can learn more about becoming a patron at Patreon.com/Scishow. And if you’re a big fan of zombie thrillers, you might like the episode on why some people like horror movies over on our sister channel. SciShow Psych. [♩OUTRO].