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Set in the 13th century middle ages and there is a zombie apocalypse, I own a castle surrounded by a trench of width 50m and a depth of 50m filled with 25m deep of mercury. The zombies are similar to the TV series The Walking Dead and I hope by now you have already seen it. It seems that I'm the sole survivor in this heavily zombie infested area. I managed to stock pile plenty of non-perishable supplies and practice rationing every day and I blew up the bridge connecting my castle to the outside world.

Would the barrier hold off the zombies? If not, what should my backup plan be if I must only die by aging (must include the use of mercury)?

Note that poisoning the zombie only serves to infuriate them more. So I guess, maybe not I don't know the show didn't tell.

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    $\begingroup$ If you don't die from zombie bites, you'll end up with a mercury poisoning. $\endgroup$
    – Alexander von Wernherr
    Commented Jan 13, 2017 at 7:15
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    $\begingroup$ Since you can't leave tracks in mercury, I think the answer is "yes, it would stop zombies in their tracks." :-) $\endgroup$
    – SRM
    Commented Jan 13, 2017 at 7:31
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    $\begingroup$ It would be very hard for the zombie to walk over it. The zombie would "slip" and fall over and be left splashing on the surface. Eventually they might dog paddle over or — Disney-lemming style — be pushed by the mass of zombies from behind. But since zombies have no metabolism to speak of there is no poisoning. And you have surrounded yourself by a moat that oozes Mercury vapours, where every zombie that comes knocking on your door is now soaked with Mercury. I would say you gain nothing from this... except possibly a sudden loss of hair, and heavy damage to your central nervous system. $\endgroup$
    – MichaelK
    Commented Jan 13, 2017 at 9:49
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    $\begingroup$ As long as you stay quiet and out of sight in your castle, the zombies have no reason to approach beyond random wandering. And for those who do randomly wander over, I assume your castle is equipped with a defensive technology sometimes referred to as "walls"? They are pretty good at keeping out organised armies of thinking men with ladders and catapults so should do well against a disorganised, uninterested collection of shambling bags of bones. $\endgroup$
    – Grimm The Opiner
    Commented Jan 13, 2017 at 11:00
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    $\begingroup$ Do you realize what 50 meters is? Around 165 feet - the height of a 16 story building. $\endgroup$
    – user11864
    Commented Jan 13, 2017 at 14:52

13 Answers 13

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You would need roughly 62500 cubic metres of mercury. One cubic metre of mercury is 13,534 kilograms. So you would need to mine 845,875 tons of mercury. In cinnabar there is 12% percent of mercury. So you would need to mine 75000000 cubic metres of pure cinnabar. In the 13th century. In 1981 we globally produced 5500 tons of mercury. Globally. With modern mining technology and with modern mercury processing.

Good luck with that.

Consider making the

  1. filling the trench with hydrochloric acid. Easier to produce, and works much better on fleshy things then mercury.
  2. You live in a castle with a trench. Just fill it with water, and second one with pikes.
  3. And for the love of everything medieval. Just use swords and halberds to kill zombies and wear chainmail and armour. You'll be fine.
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    $\begingroup$ Extra protection if your water trench has bitey fish. You many not be able to get piranha but there are plenty of fish that can do a lot of damage to a zombie with the time it takes for a zombie to traverse that much water. $\endgroup$
    – Overthinks
    Commented Jan 13, 2017 at 11:21
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    $\begingroup$ Also, you might mention that the real point of moats in the middle ages wasn't to prevent people crossing over it, it was to prevent people from tunneling under the walls as attempts to dig under it would result in tunnel collapse. $\endgroup$
    – Shufflepants
    Commented Jan 13, 2017 at 14:58
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    $\begingroup$ @Overthinks You seem to have overlooked the distinct possibility of Zombie Piranha. The distinctly terrifying possibility of Zombie Piranha. $\endgroup$
    – Sidney
    Commented Jan 13, 2017 at 15:31
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    $\begingroup$ @Sidney I really don't understand. In what way would Zombie Piranhas be more dangerous to living humans than regular Piranhas? This isn't Dwarf Fortress where undead fish can walk on land. $\endgroup$
    – Random832
    Commented Jan 13, 2017 at 22:01
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    $\begingroup$ @Random832 zombie piranhas wouldn't die if they got out of the water. They'd just flop along on land. And keep spreading. $\endgroup$
    – SRM
    Commented Jan 14, 2017 at 9:41
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No

Mercury wouldn't affect zombies any more than water would do.

It'll affect you far more as mercury vapour is toxic, and you're surrounding yourself with a lake filled of the stuff...

Any zombies that manage to traverse the moat will also bring a payload of mercury closer to you.

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    $\begingroup$ This seems like a strictly worse version of "shambling, flaming zombies". Instead of them being more dangerous because they are on fire, they are much more deadly because even after being put down, you have to dispose of all of that mercury $\endgroup$
    – D.Spetz
    Commented Jan 13, 2017 at 15:54
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    $\begingroup$ Yes, the vapours are really, really toxic, but as I mentioned in the comments to the question: Mercury vapour is not really a problem. Mercury has a vapour pressure of <1 Pa at 20°C, compare this to water (2.3 kPa) and ethanol (5.8 kPa) which both evaporates quite readily. Yes, there will be some Hg evaporating, but the amount is negligible as long as you don't heat it up to boiling. People getting poisoned by mercury vapours have either been working with vacuum devices or with hot mercury. The lake sized amount might give a substantial amount of vapour, but he'd be dead getting it to the moat $\endgroup$
    – Mrkvička
    Commented Jan 13, 2017 at 16:09
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Besides all the other considerations, Mercury is highly dense, meaning it is fairly easy to float on it... and though your zombies may be clumsy they may still figure out how to increase their shoes footprint to simply walk over the trench.

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    $\begingroup$ This. Density of flesh is approximately equal to the density of water, or 1 g/mL. Density of mercury is 13.56 g/mL (I think, according to Google). The zombies won't get more than knee deep. In fact if they can't walk across they could probably just roll or belly crawl across the surface. $\endgroup$
    – Scott Whitlock
    Commented Jan 13, 2017 at 17:28
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    $\begingroup$ @ScottWhitlock i'm thinking a weird kind of slip-n-slide dynamic would carry them over the moat given even a small nudge. $\endgroup$
    – teldon james turner
    Commented Jan 13, 2017 at 18:08
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    $\begingroup$ The bridge is probably sitting on top of the mercury, too. $\endgroup$
    – kettlecrab
    Commented Jan 13, 2017 at 20:44
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It would at least be worse than just an empty trench/moat, or even a water-filled moat. Since Zombies would likely float, even if they wouldn't be able to swim, they'd still be able to crawl over one another, and then would only have to build a pyramid ~30 meters high (counting for around 5 meters of zombie-debris being pushed down into the mercury - it could well be less) to climb out on the other side.

While this sounds difficult and challenging, it's less challenging than simply having an empty moat. Then the zombie-pyramid would have to be 50 meters high.

I'll leave it up to the mathematically inclined to calculate the volume and number of zombies required to "bridge" each gap, but I expect the difference to be around a factor of 2-3, simply because a higher pyramid will need a much larger base. How much larger depends on the compressible strength of a zombie.

In fact, since zombies cannot build bridges longer than a few zombies (their tensile strength isn't very high), the key element to protection using "void" is to make sure that they (or anything else) cannot fill this void. My proposed solution would be to create a rapid flow of water at the bottom of the void to wash any debris (including zombies) away.

Build your castle next to a big waterfall, and carve a moat that will act as a canal for the river. Then divert the river through the canal and excavate the old riverbed to have a similar profile to your canal. It should then be well beyond any but the largest amount of animated corpses to cross your moat.

Additional defenses would have to target the tensile strength of a mass of zombies. Pour some soap, oil or drain cleaner into the water upstream of the zombie-crossing, to weaken inter-zombie tensile-strength. Pour light oil into the water and light it aflame, to weaken intra-zombie tensile strength (assuming burnt, rotten flesh not being as flexible and more brittle than just rotten flesh).

This would also be more compatible with early medieval capabilities ;)

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    $\begingroup$ I have always thought with the walking dead that trip wires and fires would be extremely useful. All of the big herds would take care of themselves(trip wire would trip them, and the they would get trampled by the zombies following behind, quickly reducing the number of zombies, and then a ring of fire with loud noises in the middle, the light and noise would attract them like moths to flame and we all know how that ends. Plus to some extent they might add fuel to the fire... $\endgroup$
    – theinvisibleduck
    Commented Jan 13, 2017 at 17:22
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    $\begingroup$ The zombies being pushed down into the mercury will also be pushed away from the pyramid. It's not the depth of the zombies that is the problem (you only need about 1:10 ratio of zombie height below the mercury line to above mercury line), it's the width you need to keep the zombies at the edge of the pyramid in place. $\endgroup$
    – Taemyr
    Commented Jan 16, 2017 at 9:39
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Just lock the door!

Mercury would have little effect on the zombies other than ensuring that their bite was even MORE dangerous to you than before.

As has been pointed out in other answers, it's not a good idea and creating that much mercury in the 13th century requires (basically) magic.

Since we are talking about the 13th century here...

Find a keep. ANY keep will work. A fortified manor house is good too. A castle gatehouse is also great. Even a single tower of a castle wall. Don't do ANYTHING ELSE, just bar the door. Sit tight, you are safer than you would be surrounded by a mercury trench! Fortifications of the time were reinforced stone walls. To climb them took considerably more dexterity than zombies are capable of. Zombies piling up enough to create a mound high enough to get over the wall would likely liquify the zombies on the bottom and simply flow AROUND it like a mud flow. They basically have no way to defeat medieval fortifications. Castles and other buildings of the time kept larders stocked with preserved food in case of siege and every castle would have an internal well.

Basically, the Zombie Apocalypse scenario in the 13th century is WAY less scary than it is in modern times. People commonly had access to (and skill with) good hand held weapons and bite proof armor, didn't need electricity, knew how to live off the land, and total population density (and thus number of zombies) was far less. I'd be very surprised if ANY 13th century castle fell to the zombie attack.

In short: forget about the mercury. Go find a castle and lock the door.

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  • $\begingroup$ If he never leaves the keep, what does he do about his sewage? $\endgroup$
    – Mawg
    Commented Jan 16, 2017 at 13:18
  • $\begingroup$ @Mawg - he has to keep it, I guess. $\endgroup$
    – Simba
    Commented Jan 16, 2017 at 14:10
  • $\begingroup$ Which might lead to some serious health problems $\endgroup$
    – Mawg
    Commented Jan 16, 2017 at 14:35
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    $\begingroup$ Castle keeps were designed to dispose of sewage and had various systems for that even in the dark ages. Every single tower would have had a garderobe or similar method for disposing of waste. The people who built those things may have been living a long time ago, but they were not idiots. $\endgroup$
    – JBiggs
    Commented Jan 16, 2017 at 14:41
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If the zombies had functioning brains they would be stopped in their tracks wondering where all the mercury came from. Does the medieval mercury industries have the capacity to produce that much mercury?

However, this is not a trench, it's a mercury filled moat and a ruddy big and deep moat at that. Quite possibly beyond the resources of thirteenth century, unless the castle owner, a liege lord by any other name, had considerable time to prepare for the medieval zombie apocalypse. Several generations would do fine.

Zombies falling into the moat will tend to float. Mercury is one dense liquid metal. So the zombies only need to lie back and float across to the castle side of the moat. What will make life, sorry "life", difficult for these zombies is not the mercury, but the twenty-five metre high moat walls. This is the height of the moat above the level of the surface of the mercury. We may neglect the twenty-five metre depth of the mercury because the zombies will easily bob to the surface.

Of course, if medieval zombies are good at climbing, then a massive mercury moat to be of no avail. Doomed by zombies! Doomed by mercury poisoning! Doomed by tertiary syphilis (see the OP's comments)! Doomed by bankruptcy due to the cost of building the mercury moat!

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    $\begingroup$ cost of up to $32,000,000,000 for a 200m long moat with todays prices $\endgroup$
    – Innovine
    Commented Jan 13, 2017 at 12:39
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    $\begingroup$ Thank Heavens! Only thirty-two trillion dollars! And I thought it was going to be expensive. :-) $\endgroup$
    – a4android
    Commented Jan 13, 2017 at 12:48
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    $\begingroup$ @a4android: BIllion, I think. Fairly cheap, considering ;) $\endgroup$
    – nzaman
    Commented Jan 13, 2017 at 16:15
  • $\begingroup$ @nzaman milliards, also $\endgroup$
    – beppe9000
    Commented Jan 13, 2017 at 19:11
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    $\begingroup$ @a4android: Nope. Nine zeros, that's American billions, unless they've changed the rules further $\endgroup$
    – nzaman
    Commented Jan 14, 2017 at 11:50
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Zombie one would be stopped by the 25m drop. The first few hundred would also be stopped. However, the mercury will support the weight of all the undead, not displacing much at all. Eventually, the mass of bodies would lodge together, stop sliding around, and begin filling in the moat. I'm not going to do the math on how many bodies it would take to fill in a 25m deep moat, but theoretically, the last zombies would simply walk across the squirming bridge created by all the others.

Ironically, the zombies bathed in mercury would take longer to rot away because of the antiseptic qualities of mercury. Now, if your zombie virus was destroyed by the mercury antiseptic factor, that would be a very different reason to want a mercury moat.

You'd probably be better off with the 50m moat without mercury. If you have to fill it with something, use lava instead. Probably be cheaper and lets face it, lava kills everything

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    $\begingroup$ A Dwarf Fortress player, hm? $\endgroup$
    – Michael Schumacher
    Commented Jan 14, 2017 at 17:00
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    $\begingroup$ @Michael Schumacher Everyone knows lava kills everything. Haven't you every thrown couch cushions on the floor and jumped from one to the next to stay out of the Lava? Also, Mario, and pretty much every video game from then up until minecraft and beyond :-) thanks for mentioning Dwarf Fortress. I may have to check it out. $\endgroup$
    – Paul TIKI
    Commented Jan 14, 2017 at 17:14
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    $\begingroup$ Note how "Bonus: use lava" is frequently mentioned in dwarffortresswiki.org/index.php/DF2014:Stupid_dwarf_trick $\endgroup$
    – Michael Schumacher
    Commented Jan 14, 2017 at 18:29
  • $\begingroup$ "The content you are trying to access has been denied by Policy (Category: "Informational;Games")." Grrrr!!! How dare they demean DF by refering to it as a "game"?! $\endgroup$
    – Mawg
    Commented Jan 16, 2017 at 13:26
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It will be slightly better than water; it is heavier and blocks light and consequently eye sight. But not probably worth it. If mercury is a must have element:

https://en.wikipedia.org/wiki/Mercury_(element)

Mercury does not react with most acids, such as dilute sulfuric acid, although oxidizing acids such as concentrated sulfuric acid and nitric acid or aqua regia dissolve it to give sulfate, nitrate, and chloride.

And

Mercury(II) oxide, the main oxide of mercury, arises when the metal is exposed to air for long periods at elevated temperatures. It reverts to the elements upon heating near 400 °C, as was demonstrated by Joseph Priestley in an early synthesis of pure oxygen.[11]

And

A mercury compound called "Mercury(II) fulminate" is a primary explosive which is mainly used as a primer of a cartridge in firearms.

https://en.wikipedia.org/wiki/Mercury(II)_nitrate

Although mercuric nitrate is not flammable it can speed up flames since it acts as an oxidizer. In addition, it can form explosive compounds when combined with alcohols.[7]

Maybe a mercury based explosives/flamethrower. Mercury is though toxic. Your main character will end up as a Mad Hatter https://en.wikipedia.org/wiki/The_Hatter

Mercury was used in the manufacturing of felt hats during the 19th century, causing a high rate of mercury poisoning in those working in the hat industry.[1] Mercury poisoning causes neurological damage, including slurred speech, memory loss, and tremors, which led to the phrase "mad as a hatter".[1]

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    $\begingroup$ Why would mercury being heavier than water help? The opposite would presumably be the case: zombies would just float on it. $\endgroup$
    – Konrad Rudolph
    Commented Jan 13, 2017 at 12:06
  • $\begingroup$ True, I thought that they would not, but in the case of floating the trench would indeed lose some of its value. Though I do not know how easy it is to "walk" on mercury as walking on it is impossible: quora.com/Is-it-true-that-you-can-walk-on-liquid-mercury $\endgroup$
    – user3644640
    Commented Jan 13, 2017 at 12:26
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    $\begingroup$ Maybe not walk on it, but a zombie would float pretty well in mercury. I think you'll have zombies end up floating/swimming across on their bellies. $\endgroup$
    – Gregor Thomas
    Commented Jan 13, 2017 at 17:13
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No

mercury wouldn't provide much more defense than the initial moat. Though the zombies wouldn't exactly be able to float across its reasonable to assume that they would be able flop across the surface and make it to the other side. Interesting video here

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  • $\begingroup$ Upvoted mostly for that video link. Some excellent mercury experiments, especially the surface tension vs. buoyancy with salt or lead shot in mercury. $\endgroup$
    – Peter Cordes
    Commented Jan 15, 2017 at 8:01
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No matter how you slice it, this strategy can only be a temporary measure, no matter what you fill it with, or how deep the canal. Zombies have infinite patience and tenacity. Every second of every day they will be honed in on whatever most captures their attention. From a practical perspective, you can consider the number of zombies as infinite, but any moat or ditch is finite, by definition. We all know that ∞ > any finite quantity. Unless your protagonist is very old, we're talking about a time scale of decades.

A huge writhing pile of groaning zombies is likely to attract even more groaning, writhing zombies. Light sources in your castle will attract them. Dislodged stonework falling from your keep will attract them. Dead animals will attract them. Your huge castle will require maintenance and upkeep after some time. Things rust, mortar loosens, treated wood will eventually develop cracks, allowing moisture and rot. And what will you do with all of your personal waste? You won't be able to hide human activity forever.

Given enough time, they will make it over or through, unless you can provide for a chasm that is thousands of feet deep.

You'll also need to consider the disease, pestilence and odor that will come off these zombies. It wouldn't take long for conditions in the immediate area to become unlivable, unless your trench is significantly distant from your castle (I would assume > 1 mile). All of the land surrounding the trench would be polluted and ruined. The groundwater would become toxic. The fallow and rotten ground would encroach little by little on your castle property.

Even the strategy of locking the door and never leaving will fail eventually. Unlike the way zombies are depicted on TV (and played by living human actors), zombies will have no sense whatsoever of personal space. They will care not about broken bones, cracked skulls, dripping entrails or any other bodily damage. As they begin finding their way to your castle wall, they will compress into each other as they bunch up against the wall. In fact, long before they find your walls, some zombies will be clumped into shambling clots of rotten, fetid, mindless animation. Over time, some of those rot-melded herds will be hundreds or thousands in size.

Eventually, as individual and clumped zombies make their way to your walls, the front lines would be pressed into splattery ichor. Over time, there will be tremendous sustained pressure against your aging castle walls. What happens when they find a soft spot? And the crushed zombies will begin forming into a pile, slowly, but eventually rising over time. They don't care how many years it takes.

A slowly growing pile of rotten and rotting flesh, right up against your castle walls.

This would be repulsive and toxic to humans, but there are certainly animals who would be attracted to this, and from many miles away. Vultures, coyotes, hawks, and nearly any starving animal will take an interest. And maggots by the ton. All of which would continue to attract zombies.

On the bright side - butterflies!

Edit: Since I don't have enough rep yet to upvote or post a comment, I just wanted to "upvote" Rathgill's answer and say thanks for quantifying the enormity of zombies! While it is true that there wouldn't literally be an infinite number of zombies, I was hoping it would be implicit that I meant that, from a lone survivor's perspective, the inexhaustible supply would be equivalent to an infinite supply over the source of their lifetime.

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    $\begingroup$ Welcome aboard! Great post. People - not enough upvotes for " And what will you do with all of your personal waste? " $\endgroup$
    – Mawg
    Commented Jan 16, 2017 at 13:28
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As others have said, no. I assume you wanted the moat to provide more resistance. If you want to weigh the zombies down I would suggest nets or thick weeds in the lake. Or an artificial swamp would do the same, higher chance they will pile up the top and crawl over the other bodies.

Or if you want to be wacky with this one, there is always custard. Non-Newtonian so an angry running zombie will stick in but be unable to pull against it.

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As other posters have indicated, you can essentially crawl across the surface of mercury. There is a photo I saw in a 1970s Nat Geo of a miner just sort of sitting on top of a pool of mercury. So I think some zombies would simply by accident fall over and start crawling. Not sure if other zombies would "get the idea."

Anyway, the moat full of Hg seems way less effective than a moat full of water or gasoline that could be replenished after being ignited. I hope you have not invested too much time/money in this already.

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    $\begingroup$ quora.com/Is-it-true-that-you-can-walk-on-liquid-mercury $\endgroup$
    – user6760
    Commented Jan 14, 2017 at 5:16
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    $\begingroup$ @user32181 Welcome to Worldbuilding! Your answer is useful. Good! Did you see the link that user6760 added in comments? Comments can get deleted, so if someone adds good commentary to your answer, it is appropriate to edit your answer to incorporate that comment. In this case, I'd encourage you to add the link to your answer since it provides citation for your claims. Bonus points if you can find that photo you mentioned! :-) $\endgroup$
    – SRM
    Commented Jan 14, 2017 at 7:16
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A previous post by Beau (https://worldbuilding.stackexchange.com/users/21766/beau) mentioned that from a practical perspective the supply of zombies is infinite. While it IS very large in relative terms it is far from infinite. I'm going to make a few assumptions to make the math quick, but if you need to change those the changes to the answer should still be easy to figure.

i'm going to assume that the zombie apocalypse happens rapidly enough that the population of the world (humans + zombies) doesn't increase significantly as zombies don't procreate, they just convert existing humans to the zombie state, and the few remaining humans don't have an appreciable birthrate post-apocalypse since nearly all of them are converted to zombies. If this last part isn't the case, then I'd argue that it wasn't much of an apocalypse, just a heck of a disruption, and things will be getting back on track shortly since the zombie horde probably is dealt with if it doesn't overwhelm humanity right off the bat. I'm also leaving out the possibility of zombie animals; their inclusion would just increase the numbers, so if you like feel free to look up the mass references and redo the arithmetic. I also round a tad for clarity, go nuts recalculating with however many significant figures you find useful.

Therefore, under the assumption that pretty much all the world's population is converted to zombies, we have 632 billion pounds of humanity on the planet according to a recent paper in BMC Public Health, summarized in this article: http://www.livescience.com/36470-human-population-weight.html

convert that to metric and you have 287 billion kg of humanity. since the density of humans (and presumably zombies) is VERY close to that of water, which is 1 kg/liter, that is 287 billion liters of zombie flesh.

The minimum square 50 x 50 meter moat needed to hold this volume is calculated by dividing by 1000 to convert liters to cubic meters, then dividing by the 50x50 cross section of the moat to get the total length, then by 1000 to convert that moat length to kilometers, then by four since there are four sides to this square moat.

287000000000L / 1000L/m^3 / (50m * 50m) / 1000m/km / 4sides =

28.7 kilometers/side or, a little less than 18 miles.

That's still a HUGE moat, but it will be a whale of a lot easier to simply dig a massive great moat like this that it would be to obtain all that mercury! And it's a long way from an infinite problem!

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