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In a given fantasy world, I need a deadly (yet not magical) gas that will naturally pool in low depressions and valleys.

The level of magic is not high, so I'd like it to be an actual chemical that an astute reader/player might be able to identify.

Ideally, it should be mostly opaque in large quantities and deadly to any unprotected living creature.

Does such a substance exist? Can it?

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    $\begingroup$ There's hexafluoride gas, but it's transparent and would sound hilarious to die in... Alternatively there's carbon monoxide. How resistant to wind should it be? $\endgroup$ – Samuel Apr 20 '15 at 16:26
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    $\begingroup$ How immediate should the deadly effect be? If all the normal air is replaced by an inert, heavier-than-air gas, people die shortly after they can no longer hold their breath. $\endgroup$ – Frostfyre Apr 20 '15 at 16:41
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    $\begingroup$ @DannyReagan It seems more evil to be invisible; insidious. I think the only reason media makes evil gas visible is so the audience has something to visualize. Also, I know a mexican place that indirectly serves evil gas. It's invisible. $\endgroup$ – Samuel Apr 20 '15 at 17:30
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    $\begingroup$ @Samuel I'm leaning towards Chlorine at the moment, because it is a nice sickly yellow; deadly, but not instantly lethal; and can plausibly be naturally occurring. $\endgroup$ – Danny Reagan Apr 20 '15 at 17:32
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    $\begingroup$ An advantage to inert gases is that humans would probably not know that they were suffocating until it is too late. The desire to breath is driven by CO2 build up. Humans readily pass out (and die) in oxygen-starved environments without having the faintest idea that anything is wrong until the last minute. $\endgroup$ – Dancrumb Apr 20 '15 at 18:33

17 Answers 17

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During WW1 chemical warfare was used to break the deadlock at the ocidental front, with cruel consequences but little to no true military value. The simplest gas used was Chlorine, that accumulated in pools and was quite deadly (and quite horrible as a weapon). I believe this one or any of the other gases used in WW1 might work as you want. Phosgene is a alternative but does not produce the thick, greenish cloud that chlorine generates. Below is a chlorine release, photo is B&W, but the cloud color is greenish (green + white).

Chlorine gas use

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    $\begingroup$ Upvoting the use of chlorine gas feels wrong, but it is certainly more opaque than hexafluoride or carbon monoxide. Good choice. $\endgroup$ – Samuel Apr 20 '15 at 16:40
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    $\begingroup$ upvoting because OP explicitly asked for an opaque gas $\endgroup$ – 16807 Apr 20 '15 at 20:34
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    $\begingroup$ @fredsbend Chlorine gas and mustard gas are not the same thing. See also Chemical weapons in World War I - "The Germans marked their shells yellow for mustard gas and green for chlorine and phosgene.." $\endgroup$ – BlueRaja - Danny Pflughoeft Apr 20 '15 at 20:40
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    $\begingroup$ I see. I didn't realize that. Thank you. $\endgroup$ – 458 Apr 20 '15 at 20:48
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    $\begingroup$ Chlorine is toxic because as soon as it gets water, it turns into a powerfull acid (HCl). Its a asphixiating agent via destruction of pulmonary tract. $\endgroup$ – Jorge Aldo Apr 26 '15 at 19:55
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Ok, I misunderstood the question, you want nasty, seeable stuff. The natural part is a bit problematic. Here we go:

  • Nitrogen dioxide
    Brownish stuff which is caused by burning which is a major air pollutant and has this sharp, biting odor. Poisonous, of course . To be exact, the atmospheric nitrogen is bonding with oxgen, first causing nitrogen monoxide which reduces himself to nitrogen dioxide. It reacts with water to nitric acid (exactly, that bad stuff) and nitric acid forms again cloud of nitrogen dioxide. Problem is, natural occurence is quite out of the line...you need too many of this stuff to form pools of nitric acid. So old mining industry or waste disposal are possible settings.

  • Chlorine
    Has already been mentioned, but here is also the problem how it is generated. The best bet is salt (NaCl) which is reduced by acid to form chlorine.

  • Acid clouds
    The most common acids like sulphuric acid and hydrochloric acid are creating fumes which are aerosols of small droplets like fog. They are not so much deadly because I cannot imagine that someone will try to enter this stuff, it is definitely a barrier.

For history here the old given invisible stuff:

  • Carbon dioxide
    Yes, the thing used in soft drinks is also naturally occuring, but poisonous in higher concentrations. It is one feared component building up in silos or wine cellars and killing more people than all other gases. There is no protection (gas masks do not work: you need an oxygen supply). At one time 1700 people were killed by the Nyos Lake.

  • Hydrogen sulfide
    While transparent, it cannot be ignored because it smells like rotten eggs (That is in fact the reason because rotten eggs produce hydrogen sulfide). It is not only very poisonous, but also flammable and explosive. It is created naturally in swamps and sewers.

  • Sulfur dioxide
    Is created when burning sulfur which can be found in nature as native element. It is colorless, very toxic, but smells like a currently lighted match.

  • Ethane
    Is besides methane the component of natural gas. While methane is lighter than air, ethane is heavier, colorless, odorless and poisonous in higher concentrations.

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    $\begingroup$ Strictly speaking, CO2 itself is not poisonous. Instead, it displaces the oxygen in the air, causing death by suffocation. Same is true for inert gasses like argon. $\endgroup$ – jamesqf Apr 20 '15 at 18:24
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    $\begingroup$ @jamesqf Suffocation from CO2 is actually distinct from inert gas asphyxiation - you probably won't notice the inert gas until it's too late, while CO2 will make you feel breathless and like you're struggling to breathe - the natural reaction is to breathe faster. Your involuntary breathing rate is actually controlled by CO2 levels in your blood. $\endgroup$ – Bob Apr 20 '15 at 19:26
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    $\begingroup$ Might want to add the $CO_2$ is also colorless, odorless, and tasteless. One of its dangers is that someone not educated in its dangers, won't realize it's killing them. $\endgroup$ – Jim2B Apr 21 '15 at 1:05
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    $\begingroup$ @Bob The opposite of inert gas asphyxiation: the med students at my old college had some class covering this where they breathed a mix of air with extra CO2 and extra oxygen. The mix was such that you could survive fine on it but because of the elevated CO2 you constantly felt like you were suffocating to death. $\endgroup$ – Murphy Apr 21 '15 at 13:02
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    $\begingroup$ @jamesqf, CO2 is poisonous. The toxicology appears to work primarily by acidifying your blood. Here's (a good resource)[blm.gov/style/medialib/blm/wy/information/NEPA/cfodocs/…. BTW, I am a volcanologist and work in caves with CO2 up to 3%. I'd like to see a reference that gives some scientific basis for the experiment Murphy reports. $\endgroup$ – foobarbecue Apr 22 '15 at 5:12
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Another possibility is Bromine. It's denser than air, so it will pool nicely, and has the literary advantage that it's red, with all the symbolism that entails. It's toxic at high concentration, but it also stinks, so it's hard to ignore and hard to imagine anyone getting killed inadvertently, as opposed to silent killers like CO2.

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  • $\begingroup$ Like the idea and the effects, but Bromine is a liquid to about 50-60 C, no? You'd have to be pretty warm to have this as a gas. $\endgroup$ – Twelfth Apr 20 '15 at 18:46
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    $\begingroup$ No, that's boiling point. You get visible gas evolved at room temperature. Just as you get appreciable water vapor in the atmosphere, even though water boils at 100 C. $\endgroup$ – WhatRoughBeast Apr 20 '15 at 19:29
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    $\begingroup$ @Twelfth Water is a liquid to 100°C, yet water vapour exists in the atmosphere. Bromine vapour can likewise exist. -edit- Ninjad! $\endgroup$ – March Ho Apr 20 '15 at 19:29
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You actually have quite a few choices. Given your parameters,

  • Argon (Ar): It’s a naturally occurring gas on Earth (it comes from the radioactive decay of a few other elements that naturally occur) and our senses can’t detect it (it’s tasteless, odorless, and invisible). It’s not really a poison in and of itself, but it replaces oxygen in low-lying areas and becomes an asphyxiation risk.

  • Chlorine (Cl): This is a bit more noticeable: argon suffocates you while you breathe normally, while chlorine causes a much more painful reaction. The gas becomes hydrochloric acid when it contacts water, and burns. Inhaling it causes an instant crippling effect and probably gives you the most visually appalling death you can think of. It does have a color and trace amounts of it become detectable.

  • Natural Gas (CH4 & others): This is actually completely scentless (the odor we recognize so well is added so we can ... well, recognize it so well). Methane (CH4) in particular can work here, though combustion might be a hazard. Inhaling it in large quantities for a short period of time cripples a human body and sets in motion a wide array of lung issues. As much as anything, it replaces oxygen in the air (or outright consumes it) and reduces what our bodies can take in.

  • Hydrogen Sulfide (H2S): This is an odd choice, but it gives you the colorless but horrible smell option. This is the rotten egg scent many of us are aware of, though it should be noted that it deadens the senses and is harder to detect than you might think. It would rather quickly discolor any copper the person might be carrying or using. It doesn’t take much: 500 part per million will completely nuetralize the sense of smell and 800 ppm will cause death within 5 minutes (and an uncomfortable one at that).

  • Carbon Tetrachloride (CCl4): This would be an interesting idea, though it’s a bit lighter and isn’t as likely to pool without a source. It has a slightly sweet smell, but is otherwise pretty undetectable in a mediveal world. It’s a horrible toxin to the liver, will place people into comas, goes after our central nervous system, and causes symptoms of depression. The deadening of the nervous system can also dissociate yourself from your body.

  • Sulfur Hexafloride (SF6): This one could be fun: it’s odorless, colorless, and not very detectable until we get to sound. It has the opposite effect to helium, lowering ones voice by several octaves (the give away that you are in it is that everyone’s voice goes lower). It’s not really toxic in and of itself: most of the death risk is from displacing oxygen and not leaving enough behind for our uses. It’s also a functioning anesthetic, just below nitrous oxide (laughing gas) in its effectiveness

There’s a pretty good array. You could probably be picky between severity and speed of reaction on the one side and color, scent, and taste on the other. If there were specific parameters that you were interested in, we could probably tailor the gas for you a bit better. Everything listed above is decently commonplace on our globe and is explainable.

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NO2 or nitrogen dioxide is toxic, heavier than air and visible (opaque) in high concentrations as your question asked. It is reactive with organic materials (life) as it is an excellent oxidizing agent. Also forms a strong acid on reaction with water (nitric acid), and ozone on reaction with volatile organics in the presence of heat and light.

Wikipedia on NO2

EPA on health effects of NO2

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My first thought is Carbon Dioxide.

It doesn't seem that deadly, but it is! (...assuming one breathes oxygen).

It has been known to pool into depressions and valleys (typically after a volcanic 'burp' or when deep water saturated with CO2 has been agitated by a distant earthquake or something).

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  • $\begingroup$ CO2 is pretty light and will not pool into depressions easily. $\endgroup$ – Jorge Aldo Apr 20 '15 at 18:56
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    $\begingroup$ wikiwand.com/en/Lake_Nyos $\endgroup$ – Bradley Uffner Apr 20 '15 at 19:04
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    $\begingroup$ Or, more generally, wikiwand.com/en/Limnic_eruption. $\endgroup$ – RomanSt Apr 20 '15 at 20:35
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    $\begingroup$ CO2 is also not opaque. You can't even smell it. This is not a good answer. $\endgroup$ – 458 Apr 20 '15 at 20:40
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Anhydrous ammonia, when subjected to moisture in air, forms a ground-clinging cold fog that rolls away from the point of dispersion and is quite noxious. It is also easier than almost anything else to make, and the final reaction after the fact tends to make the grass a little greener. Are there bonus points for being eco friendly?

Oh, and it's explosive.

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This is a real story about a lot of people dying.

Lake Nyos is in a volcanically active area of Cameroon. It sits over a pool of magma, which leaks carbon dioxide gas into the lake. The gas dissolves in the water, but the lake becomes supersaturated. That means that any sudden disturbance of the water (caused by a small earthquake or landslide, for example, which is not unlikely in a volcanic region), can cause it to suddenly release a lot of gas.

A massive cloud of carbon dioxide pouring down a hillside can kill a lot of people.

Wikipedia says,

On August 21, 1986, a limnic eruption occurred at Lake Nyos which triggered the sudden release of about 100,000 - 300,000 tons (some other sources state as much as 1.6 million tons) of CO2; this cloud rose at nearly 100 kilometres per hour (62 mph). The gas spilled over the northern lip of the lake into a valley running roughly east-west from Cha to Subum, and then rushed down two valleys branching off it to the north, displacing all the air and suffocating some 1,700 people within 25 kilometres (16 mi) of the lake, mostly rural villagers, as well as 3,500 livestock. The worst affected villages were Cha, Nyos, and Subum. Scientists concluded from evidence that a 100 m (330 ft) fountain of water and foam formed at the surface of the lake. The huge amount of water rising suddenly caused much turbulence in the water, spawning a wave of at least 25 metres (82 ft) that would scour the shore of one side.

Carbon dioxide, being about 1.5 times as dense as air, caused the cloud to “hug” the ground and descend down the valleys, where various villages were located. The mass was about 50 metres (160 ft) thick and it travelled downward at a rate of 20–50 kilometres per hour (12–31 mph). For roughly 23 kilometres (14 mi) the cloud remained condensed and dangerous, suffocating many of the people sleeping in Nyos, Kam, Cha, and Subum. About 4,000 inhabitants fled the area, and many of these developed respiratory problems, lesions, and paralysis as a result of the gases.

If you want coloured gas, you’ll need to use something other than carbon dioxide. However, do check whether the gas is water-soluble. If it is, a lake over a vent can cause a natural way for a lot of gas to gradually build up over a long time and then be suddenly, unpredictably released.

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    $\begingroup$ Nyos was my first thought when I read the question. A possibly interesting side-note is considering this effect as the mechanism for the death of all the first-born during the original passover in the biblical book of Exodus. The first-born slept in a place of honour on the ground floor. $\endgroup$ – Ray Butterworth Mar 22 at 13:16
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There exist completely naturally occurring gases which satisfy the criteria. I remember that in the Transylvanian mountains there are locations where sanua-like buildings are built upon such places, where the gases have the following properties:

  • heavier than air, doesn't mix with air.
  • considered healthy to the bones, joints, etc., good against arthritis and similar illnesses.
  • heavily toxic. One breath can easily kill you.

You can check the level of the gas with a cigarette lighter or a matchstick. If you lower the flame below the level of the gas, it goes out completely. The gas is completely invisible, and feels a little warm. If you carefully enter the "sauna" and sit down, you can be almost neck-deep in the gas, and completely safe.

Tourist guide quote, emphasis added:

What is a mofette? Usually tourists don’t know the meaning but you have to know that this is one of the many advantages that Sekler land has. It’s a special bath without water. The gas is coming out of the earth and as it gets into the blood-stream through the skin it starts its healing process. These gases come up from the result of post activity of the volcano and they are present in many places all over Sekler land. The types of gases are usually carbon dioxide and sulphur. It is good for circulation problems, blood-pressure problems and for motorical problems. Of cause you need to have a few day treatment if you want any results. The mofette can be very dangerous too. Breathing in these gases can be fatal. We have to be careful, do not bend under the gas sign line and do not mix up with sudden movements.

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  • $\begingroup$ What is the gas that you mention? (makes for an interesting addition to the setting that people use it but it is deadly) $\endgroup$ – MER Apr 21 '15 at 7:26
  • $\begingroup$ I've added a tourist guide quote specifying the gases as "usually carbon dioxide and sulphur", but i couldn't find a source for the claimed danger (i.e. no mofette deaths). $\endgroup$ – Cees Timmerman Apr 21 '15 at 7:47
  • $\begingroup$ @CeesTimmerman : There is little I could offer besides anectotal evidence, because these mofettes are not that well-known, and no one famous enough wrote about them in the English-speaking media. These are not tourist attractions with a large tourist industry built around them, they are just used by the local villagers and whoever happens to hear about them. It might be an exaggeration that "one breath can easily kill you" but i wouldn't risk my life to try it. I guess it can happen if you are alone, one breath could make you faint and then you die if no one is there to drag you out in time. $\endgroup$ – vsz Apr 21 '15 at 18:38
  • $\begingroup$ I've heard about suicides involving mofettes, I guess accidental deaths are uncommon because few people besides the locals know about them and they are very careful. Also, the gas is heavy, and doesn't mix with air, if you turn a cigarette lighter to maximum and lower it below the level, you can clearly see that only the "top part of the flame" burns. If you disturb the gas, you can see waves just as in water, of course, only with the help of lighters, because the gas is completely invisible. $\endgroup$ – vsz Apr 21 '15 at 18:42
  • $\begingroup$ The only safety mechanism (besides wooden rails to keep your from slipping and falling while sitting down or standing up) is that there is no door, and the door-frame is built in such a height that its bottom lies at neck level when you sit down inside, so any excess gases can flow out. $\endgroup$ – vsz Apr 21 '15 at 18:49
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Radon

Radon is a very heavy radioactive gas. It is naturally present in low concentrations almost everywhere in the world airs and in underground waters, but it is very hard to obtain in large quantities and will decay pretty quickly in a few days (isotope 222, the most long-lived, has a semi-life of a few less than 4 days), so you will need some way to replenish it. But, anyway, it surely do what you want:

  • Tends to pool down on depressions and lower areas where it can't escape, like underground rooms, basements and mines. In fact, Radon is the heaviest naturally-occurring gas.
  • In large quantities might suffocate creatures by displacing oxygen. Even if you can handle that, or if the concentration is not so high but still many times higher than natural level, its intense radioactivity will kill any unprotected living creature. If some creature survives the exposure, it still is very likely to die from cancer some time later.
  • Even when 222Rn decays to 218Po in less than 4 days, then that also decays in a few more than 3 minutes to 214Pb that decays in 27 minutes to 214Bi, which decays in 20 minutes to 214Po, which decays in some microseconds to 210Pb which in 22 years became 210Bi, which in five days become 210Po, which in 138 days becomes the finally stable 206Pb. The result is that your radon gas will quickly fill up with highly radioactive, heavy and toxic metallic dust. Further, that dust will tend to stick to surfaces and be dissolved in liquids, making it very hard to remove the radioactivity even if all the Radon gas is ventilated away.
  • It would be almost invisible. Being a noble gas, Radon is tasteless and odorless. It is also colorless. However the mostly Lead (with a bit of Polunium and Bismuth) dust will not be, making it somewhat noticeable as a (likely gray, but not sure) smog. The smog will probably be very thin because Lead dust do not tend to be suspended in the air for long time and will soon accumulate in the ground. If there are some strong air currents in the place that do not disperse/leak the gas away, but do not allows the dust to settle down, then the smog will become quite noticeable.
  • Even if the gas is invisible and the smog normally almost invisible (if you do not prevents the dust from setting down), an astute reader/player might still be able to identify it by the accumulating Lead-rich dust. Then, after identifying it, the player will die due to the radioactivity in a few minutes. You don't need to make the gas opaque and visible for it being noticeable, the accumulated/accumulating dust might be enough.
  • Even most of digital machines, like smartphones and computers, are very likely to malfunction and glitch in such environment if not properly shielded from the radiaction.
  • Unaware normal people will likely not understand what is horribly wrong with the place/places where the Radon is. For them, it will looks like some sort of terribly strong evil magical curse.
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    $\begingroup$ I have a background in nuclear power. You are breathing radon right now. I promise. $\endgroup$ – Sean Boddy Apr 21 '15 at 8:23
  • $\begingroup$ @SeanBoddy Yes, I know. It is responsible for most of the natural background radiation and is present in small concentrations in air (specially over continents except Antarctica, where almost everyone lives) and is also present in underground waters. In fact, I even possesses a bottle with monazitic Thorium-rich sand that I grabbed in a beach and there should be a higher than usual (but still safe) amount of Radon there. $\endgroup$ – Victor Stafusa Apr 21 '15 at 9:38
  • $\begingroup$ As I understood the question, invisible gases are not wanted, but visible (opaque = intransparent) ones. $\endgroup$ – Paŭlo Ebermann Apr 26 '15 at 19:01
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I think ozone might serve. It is roughly 50% heavier than air and is known to collect in basins like Los Angeles.

Most people will recognize the chemical by smell and they probably know that it is poisonous.

Ozone itself is a pale blue, however lethal concentrations are far far below what would be considered opaque. It would be visible in the form of smog when combining with organic molecules in the atmosphere. Sometimes smog can be rather opaque. If you had the right combination of organics naturally in your atmosphere, the smog could be the opaque component though it's the ozone that kills.

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Many welders have been killed by Argon Gas suffocation...As it pools in low areas especially in concrete foundations where it cannot escape...

EDT: Argon is an inert shielding gas meaning when it is used for welding it shields the molten metal from exterior gases such as oxygen...It is used in Metal Inert Gas welding or MIG welding. It is odorless unless an odor is added such as garlic or onion fragrance. It is clear and it kills indiscriminately by starving the subject of oxygen death occurs relatively quickly usually under 3 min

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    $\begingroup$ could you expand this answer a little bit? It's a good start. $\endgroup$ – bowlturner Apr 20 '15 at 18:29
  • $\begingroup$ Argon is an inert shielding gas meaning when it is used for welding it shields the molten metal from exterior gases such as oxygen...It is used in Metal Inert Gas welding or MIG welding. It is odorless unless an odor is added such as garlic or onion fragrance. It is clear and it kills indiscriminately by starving the subject of oxygen death occurs relatively quickly usually under 3 mins. $\endgroup$ – user2218260 Apr 21 '15 at 1:34
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Carbon Dioxide.

We have valleys with this on earth, they are called Mazuku, and have been responsible for deaths of humans, wildlife and lifestock both in recent history (e.g. Lake Nyos in the 1980s or smaller incidents like Mammoth Mountain, California in 2006) and in the more distant past (perhaps the reason that the Messel Pit in Germany has so many fossils is such an event happened there).

It's not visible though, but that could be solved by a visible effect being part of the world building.

If you really need to be able to see it, I'd go with high concentrations of chlorine, though note that it's likely to be poisonous in much lower concentrations than make it visible, so your characters can't just walk up to the smoky yellow-green clouds unaffected.

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My I ask why it has to be opaque? If it's so characters can recognize it, consider that seeing the gas is not the only way to recognize the existence of a deadly gas. Seeing the effects it has on others is a key way we do so in the real world.

I ask/say the above because the first thing that came to mind was Sarin. It is heavier than air as you request, it's quite deadly, and the effects on living things is recognizable to a learned observer. It is everything you seek except the opacity.

I would venture to say Sarin is a better killer due to its transparency; you don't know it's coming until you or someone close to you can't breathe anymore, at which point you're pretty much screwed.

If you're in the U.S., "60 Minutes" had a story on Sarin just two days ago. Go watch it.

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Nitrogen. It is deadly if it pools in a low place. Before you know it, you pass out and die. Nitrogen is odorless and colorless. It is used to flush explosive gasses from pressure vessels and other containers used in industry. Sometimes a worker will go into the container to perform repairs to the vessel. He gets lightheaded, faints, then dies very quickly. There are cases where others go in to save the worker, and they die, too. The rule is, when you see a worker who is down and you don't know why, you don breathing gear.

Nitrogen is slightly lighter than air (which is itself 70% nitrogen) but I think it could be what you're looking for.

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I seems to me that chlorine gas on lower elevations would lead to highly acidic rain rendering the higher elevations uninhabitable as well so you'd probably want something that doesn't combine with water with such enthusiasm. Inert gases are probably the better option even though they have an annoying tendency to be transparent. And it doesn't really matter which inert gas it is since the effects and appearance would be all the same

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All gases mix completely with all other gases. At least we haven't yet found a pair that act like oil and water.

So it will need to be regenerated somehow or it will diffuse away, especially if exposed to wind or other currents.

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    $\begingroup$ Hi Bloke Down The Pub, and welcome to Worldbuilding. Since especially the first half of this answer goes against several already posted answers, it would be beneficial if you can provide some kind of citation for your claim beyond "that's how I say it is". That is probably why this answer has been downvoted. $\endgroup$ – a CVn Mar 22 at 6:28

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