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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?
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).
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.
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.
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.
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.
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).
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.
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.
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:
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.
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:
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.
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
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.
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.
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.
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
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.
