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I am a DM for a homebrew D&D 5e game. In terms of technology, the time period loosely takes place somewhere between 1100 and 1400 CE (so High Middle Ages to Late Middle Ages).

The villain of my piece knows of some substance (TBD) underground that, when ignited, would destroy everything within several miles of the ignition point. The problem I am running in to is I would have no idea what kind of substance that would be.

I was thinking it could be some kind of naturally occurring gas, but I am not sure if something like that exists, or exists but is not concentrated enough to be able to cause the amount of damage I would need it to. I should also mention that the substance needs to be something that has existed and/or still exists in the real world. I don't want magic to have anything to do with the villain's scheme.

Does anyone know of a substance that could fulfill the requirements that I need it to?

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    $\begingroup$ does it need to destroy if fast or can it take years. $\endgroup$
    – John
    Nov 13, 2021 at 1:11
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    $\begingroup$ Is it acceptable to destroy everything in one direction from the ignition point, or does it have to be a circle centered on the ignition point? $\endgroup$ Nov 13, 2021 at 2:57
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    $\begingroup$ How long has the villain had to work on this? $\endgroup$
    – Ash
    Nov 13, 2021 at 4:33
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    $\begingroup$ @John- The destruction would need to be fast. $\endgroup$ Nov 13, 2021 at 15:42
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    $\begingroup$ Underground -> there is no air. Some ventilated tunnel system filled with coal or wood might work, but it would be more unusual in a medieval setting than using explosives (which were already known, but were not used). I think the realistic answer to your question is that there is no such thing and I disagree the upvoted answers below your question. $\endgroup$
    – Gray Sheep
    Nov 14, 2021 at 21:00

16 Answers 16

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If it is the aim of the villain to make a trap or to make a bang big enough to destroy a town, then a dust explosion could easily fit the description.

A villain might reasonably know about the potential of dust explosions if they were a miller or might have spoken to one who told the villain about flour explosions, or a miner who might be aware of coal dust explosions.

By suspending bags of finely-milled coal or flour near the top of a large underground space, then rupturing the bags (the more violently the better) in the presence of a source of ignition, a fuel-air explosion might be caused. By sealing any exits prior to ignition, with sufficient fuel and air, pressure might build up inside the underground space to the point where the ground could rupture due to the overpressure.

This could cause significant destruction of surface structures, either due to the ground rupturing, or due to collapse of the roof of the underground space.

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    $\begingroup$ Apparently they're so common they're 'not worth writing home about', until you level an entire town that's 4 miles away, heard 500 miles away, and shatter windows within 85 miles : DuPont Powder Mill Explosion, Pleasant Prairie, Wisconsin On 9 March 1911. "300 tons of dynamite, 105,000 kegs of black blasting powder, and five railroad cars filled with dynamite" - "Most buildings in a 5-mile radius were rendered flat or uninhabitable" $\endgroup$
    – Mazura
    Nov 13, 2021 at 23:00
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    $\begingroup$ Or lumberjacks, farmers, or botanists : 'coal, sawdust, grain, flour, starch, sugar, powdered milk, cocoa, coffee, and pollen.' $\endgroup$
    – Mazura
    Nov 13, 2021 at 23:31
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    $\begingroup$ Granary explosions have been known since ancient times. $\endgroup$
    – Monty Wild
    Nov 14, 2021 at 4:34
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    $\begingroup$ @Mazura You've described the explosion of 20th century explosives manufacturing plant, which doesn't seem terribly relevant here. $\endgroup$ Nov 15, 2021 at 15:36
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The most obvious real-world option would be a coal-seam fire. It's not going to inflict fast destruction, but it could make a wide area uninhabitable for decades. For example, the Centralia mine fire has been burning for almost fifty years, rendering a couple of towns uninhabitable, and will likely run for many more.

It's easy to find flammable gases underground - decomposition produces methane, for instance. The obstacle to creating an earth-shattering kaboom is that you'd need to have the right mix of flammable gas and oxygen. In real life that's unlikely to happen at the kind of scale you're looking for, but for D&D you could handwave it a bit; maybe some local fungus has created just the right mix to be dangerous.

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    $\begingroup$ A coal seam fore would definitely do it but it would be slow destruction - with a capital S! $\endgroup$
    – Mon
    Nov 13, 2021 at 1:36
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    $\begingroup$ @Mon Yeah, really depends on whether the villain is up for delayed gratification! $\endgroup$
    – G_B
    Nov 13, 2021 at 3:12
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    $\begingroup$ @GeoffreyBrent maybe the villain doesn't as much want to see the world explode, they just want to see it burn. $\endgroup$ Nov 13, 2021 at 10:25
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    $\begingroup$ It would be slow, but on the other hand, burning methane or something would be one big fire with everything recovering quickly afterward. A gas explosion would be something grandparents tell kids about in stories. A coal fire would have grandparents warning those kids to stay away from the ruined land that might kill them if they enter it. $\endgroup$ Nov 13, 2021 at 13:24
  • $\begingroup$ Coal-seam fires can burn for a very long time. Burning Mountain in Australia is a coal-seam fire that's been going for roughly 6000 years and advances by about a metre per year. $\endgroup$
    – Rohan
    Nov 14, 2021 at 9:15
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If the area is like Paris, in that much of the rock has been quarried away from beneath it, then firing a few of the remaining support pillars using simple brush wood would compromise the strength of the remaining rock and cause widespread collapse. This is similar to the old practice of undermining where tunnels were dug under walls and carefully supported until they wanted them to collapse, in this case the tunnels were made long ago for a different purpose.

The other obvious answer is salt; lenses of rock salt can be vast and they can be dissolved quite rapidly, if your villain knows of an old salt mine that extends under the area then a small explosion that allows water to flood into said mine would destabilise everything sitting above the salt dome.

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    $\begingroup$ This is some seriously stuff. I encourage doing some research on Le Rue D'Enfer - literally Hell Street - which collapse in a scenario exactly like this. $\endgroup$ Nov 15, 2021 at 4:11
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A Forest.
Or a city built of wood.

Those are perfectly correct answers to "What kind of substance that existed in the middles ages could, when ignited, potentially destroy everything within several miles?"

Bear in mind that "several miles" requires something with more energy than even a small tactical nuke. No single explosion, fuel-air bomb, or dust explosion will come close to that requirement, by several magnitudes.

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As other answers have suggested a fuel - air explosion is your best bet. This is the technology used in the most powerful non-nuclear weapon in use. The question is what is the source of the fuel. I think the most realistic way to produce the scale you are talking about, though very unlikely, would be natural methane emissions from methane hydrates.

As the climate warms, methane is being emitted from various methane hydrate deposits around the world, most notably in Siberia. One such is described here:

Reindeer herders in the area of Seyakh, a village on the eastern coast of the Yamal Peninsula in the morning of June 28 reported the incident to the local authorities. They say that they from the distance saw flaring flames and a column of smoke from the area.

Not the sinkhole in question Not the sinkhole in question

It is quite conceivable (though unlikely) that it could naturally be released at such a scale that a fuel-air explosion could happen at any scale. How your villain causes this on demand is a question. Perhaps they divert a river toward (thus melting the hydrates) or away from (thus reducing the pressure on the deposits). Perhaps they cause a cave collapse, causing enough disruption to cause breakdown of the hydrates. Perhaps they cause a smaller explosion, that again causes the disruption. There are suggestions that drilling can cause these explosions, "leaks from production facilities may have led to the forming of ‘gas pockets’ and consequent eruptions". It is going to be challenging to do this scale of drilling with middle age technology, but perhaps with a bit of hand waving that could work.

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    $\begingroup$ agree. There are aplenty options for an accelerant (correct for the ignighted substance?) But what mechanism to use to mix oxygen to the fuel? Being a D&D world... hah... summon a major air elemental right in the middle of the mix. And hope it doesn't have friends that will hold a grudge when its blown to smithereens? But maybe the OP want a more earthy mundain mechanism. $\endgroup$
    – Gillgamesh
    Nov 13, 2021 at 14:19
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    $\begingroup$ Methane,+1. If you just want to kill everyone like a neutron bomb, use CO2 : Lake Nyos disaster 'An eruption that triggered the sudden release of about 100,000–300,000 tons (1.6 million tons, according to some sources) of carbon dioxide killing 1,746 people and 3,500 livestock.' $\endgroup$
    – Mazura
    Nov 13, 2021 at 23:42
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Greek Fire is one man-made option. It was a mysterious and terrible incendiary weapon used by the Byzantines in roughly your period. It's perhaps not quite as powerful as you need, but could make for a good story because all we really know about it are educated guesses by Medieval historians and awed witness reports. This would allow you to tailor it a bit to what your campaign needs.

By some accounts, it was powerful enough to torch a fleet, and only burned stronger in contact with water. Your character might know of a vast underground cistern filled with Greek fire, or of a way to feed it into a city's sewers or aqueducts.

The Wikipedia page links to the main historical sources. Some possible pieces of inspiration from fiction: C.J Sansom uses Greek fire in one of her Shardlake novels, which are set close to your time period. Raymond Feist has something similar to it burn down a city by firing its sewers, in his third Serpentwar book. I haven't linked to it here to avoid spoilers.

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    $\begingroup$ Welcome to worldbuilding. Please take our tour and refer to the help center about how we work. Nice answer, I'm surprised no-one posted this before, especially given the scenes in the popular Game of Thrones ("wildfire"). Enjoy the site. $\endgroup$ Nov 15, 2021 at 4:26
  • $\begingroup$ greek fire might destroy a block or two but even if you had a football stadium of the stuff it is not going to destroy things for miles. $\endgroup$
    – John
    Nov 18, 2021 at 0:05
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Your problem is that there's no naturally occurring chemical chemical compound I can think of that will produce the effect you want (total destruction) quickly. As Geoffrey suggested a coal seam fire will do it but its a slow process. So if the villain's aim is just to ruin the townspeople not necessarily kill them that would do the trick.

Beyond that? Something like a natural gas explosion could be done and could be quite violent but they are very localised.

One possible solution however might be a coal seam gas explosion in a coal mine! But it would basically require that your town is built over a deposit of coal and that historically a large part of it's living was made by mining it, even if that's not so much the case now. For a start the seam would have to be close to the surface for the medieval miners to exploit it. Then the level of engineering available would mean lots of narrow, poorly ventilated mine tunnels running under the village.

You might have also posit the existence of a geologic fault, maybe a large underground chasm (or series of smaller chasms) where gas could accumulate over time that the villages had discovered while mining long ago and then sealed off because it was easier and safer to work around than try and work in.

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  • $\begingroup$ If we are in the business of just ruining the town, then we could chuck some natural breeder reactors in the explosion to make a rudimentary dirty nuke. $\endgroup$ Nov 13, 2021 at 12:44
  • $\begingroup$ I suppose geology could put a high grade uranium ore deposit on top of a coal deposit bit I've never heard of it happening. $\endgroup$
    – Mon
    Nov 13, 2021 at 21:58
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Right, coal or more specific coal dust. Look up dust explosions or fuel air bomb All you need is a confined area a source to allow oxygen into the area and a fine dust or particle mist. Even these or gas explosions won't produce devastation above ground in a miles wide area. But may be enough to collapse some areas or create blowouts of hot gases or other nastyness. More directly to your scenario I possible plotline could be a fungus infests a large cavernous area. Normally another creature would feed on this fungus to keep it under control. However the evil villain has exterminated the controlling orginisim, leting the fungus grow wild. In a short time the fungus will mature mass release its combustible spores.

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The secret ingredient which your villain has (that others do not) is knowledge. He somehow knows that beneath the soil layer of these lands, beneath a thin layer of water-impregnable igneous rock, there lies an enormous deposit of calcium carbide. This substance, when exposed to water, releases acetylene gas.

To decimate all life for miles around, he need only bore a hole down through the igneous rock, then divert a nearby river into the opening. The resulting geyser of flammable gas then only needs a spark to ignite and once that conflagration back-burns into the hole, the resulting explosion will shatter the igneous rock shell, making the surrounding lands subject to spontaneous acetylene fire whenever the rains fall.

Could this happen in the real world? Probably not. But it is chemically valid, so that is probably good enough for a D&D scenario.

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So you probably can't find an explosive material from the middle ages that would cause that much devastation, but the Roman's destroyed large swathes of land using a form of hydraulic mining technique that Pliny the Elder described in 77 CE. The evidence of that technique can still be seen today in Spain at Las Medulas and other spots in Europe.

https://en.wikipedia.org/wiki/Las_M%C3%A9dulas

So maybe the villain uses gunpowder/black powder (9th CE China, the medieval formula was decaying matter, urine, and oxygen closer to nitrogen crystal fertilizer) to set off a chain reaction that causes a devastating flood that would wipe out a large swath fairly easily. Maybe targeting an aqueduct, or diverting it to fill up this large basin similar to what the Roman's did would serve your plot.

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    $\begingroup$ Welcome chubbs. I think that this qualifies as what we refer to as a "frame challenge", not an easy thing to do right. Not bad for a first post. Please take our tour and refer to the help center for guidance as to our ways. Enjoy the site. (From review). $\endgroup$ Nov 14, 2021 at 6:00
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Perhaps your villain discovered a natural nuclear fission reactor, having one such reactor causing an explosion is unlikely to happen naturally. However, maybe your villain discovered a deposit which just needs a "little" push to get it critical.

I put little in quotes because the size of that push might be larger than the resulting reaction, but, with a little bit of handwavium maybe that push is smaller for this particular deposit.

If we can accept that: Then it would take significant preparation and time for him to set up this explosion. Plenty of time for a band of heroes to hear about his plot and venture out to stop his evil deed...Looting everything that isn't nailed down along the way...

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  • $\begingroup$ It's fair to say that there is no realistic scenario where you could create a nuclear explosion without building an actual nuclear bomb. Nukes work by slamming greatly purified plutonium together (using conventional explosives) to bring them into critical density and mass to explode. It doesn't really matter how much nuclear material you can get your hands on, without the ability to compress it together at hypersonic velocities you won't get an explosion. But you could certainly kill a lot of people with the uranium itself if you wanted. Radiation Poisoning is no joke. $\endgroup$
    – Ruadhan
    Nov 15, 2021 at 11:04
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One box containing nuclear grenades (manual included)

Q: "The villain of my piece knows of some substance (TBD) underground that, when ignited, would destroy everything within several miles of the ignition point. The problem I am running in to is I would have no idea what kind of substance that would be."

This assumption of your villain being aware of the destructive force poses an issue for your story. Medieval people were not aware of the possibility of any weapon of mass destruction.

When the TBD object is first discovered, your medieval character will have no idea what to do with it. The knowledge about the destructive force can only be there, if he was able to activate and test it.

Now suppose.. he found a small stock of 80kT nuclear grenades, left (or hidden !) by 24th c. time travelers. With the stock, a small manual is provided and warnings about the apocalyptic effects. Each grenade has a time clock, enabling to get away safe, before the thing goes off. Your villain character got curious and took one of these grenades to an uninhabited mountain region. He followed the manual and tested it, so he knows what the weapon can do.

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Black powder existed since before the middle ages, although it wasn't known in Europe for some time. The only limit on how big a boom is how much time/money you can spend on making it.

Destroying a whole city will take a lot of powder but it could be done.

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Does it have to physically destroy stuff or is suffocating everyone in the area good enough? If so, find a way to trigger a Limnic Eruption. A small landslide could be sufficient.

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Your villain loves insects.

https://en.wikipedia.org/wiki/Bombardier_beetle

In your world, you can have a variant that is slightly more explosive or larger.

However, this is a hypergolic reaction, not one that needs to be ignited. Just two liquids that contact each other.

He has as much time as needed to breed them and has developed a method of extracting their explosive organs and storing them. Given time and quantity, you have enough to do serious damage. Pack it into a shallow rocky cave and you can get your explosion.

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Lava

It's a substance that is underground.
It can be incredibly destructive.
It ignites most of what it touches. Rather than 'ignite it' the villain evokes or triggers a lava flow or an eruption.

Beyond the infamous Pompeii eruption in the Roman era, Mount Vesuvius erupted after WW II and destroyed some local towns around Naples. (My source on that is a book Naples '44 by Norman Lewis that I no longer have access to).
There is ongoing lava flow from Etna, which is on Sicily, in the news now and again.

Since I also play D&D 5e, I'll offer to you that your villain can get access to the level 8 earthquake spell (druid/cleric) and use that to create deep enough fissures, or a fracture/fissure in just the right place, to trigger a locally risky fault zone that either starts oozing lava, or erupts, based on how you want to narratively implement the Lava hazard. Do you want slow and inexorable, or fast and furious? That's up to you as the DM.

You can also, as the DM, establish that the villain is the only one who knows how close to the surface the lava zone is, such that the calamity comes as a surprise to the local inhabitants. You have a lot of latitude in the 'how' of implementing a natural disaster.

Examples close to your time frame: Monte Nuovo or Vesuvius

I used to live in the Campi Flegrei region of Italy.

Monte Nuovo ("New Mountain") is a cinder cone volcano within the Campi Flegrei caldera, near Naples, southern Italy. A series of damaging earthquakes and changes in land elevation preceded its only eruption, during the most recent part of the Holocene, which lasted from September 29 to October 6, 1538, when it was formed.2 The event is important in the history of science because it was the first eruption in modern times to be described by a large number of witnesses.[3] The eruptive vent formed next to the medieval village of Tripergole on the shores of the then-much larger Lake Lucrino. The thermal bath village, which had been inhabited since ancient Roman times and was home to notable Roman-era buildings including Cicero's villa, was completely buried by ejecta from the new cinder cone. Tripergole's ruins and its important thermal springs completely disappeared under Monte Nuovo such that the exact location of the village can no longer be identified.[4]

And then there's Vesuvius again...

In 1631, Mt. Vesuvius gave vent to a powerful eruption. By all accounts, it was a highly explosive event that rivalled in intensity the famous eruption that doomed Pompeii and Herculaneum in the first century a.d. Sources say that the eruption destroyed most of the towns in the area of Vesuvius. The event was so terrifying that it stoked the creative imaginations of the great painters of the day, primarily > Micco Spadaro (name in art of Domenico Gargiulo, 1610-75). His "Eruption of Vesuvius in 1631" (painting, right) shows the procession of the populace, viceroy, church prelates and aristocracy. They carry the bust of the Patron Saint, Gennaro, in a show of penitence, invoking divine mercy.

Given that this is set in D&D 5e: magic exists.

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  • $\begingroup$ If we ignore the fact that lava can't be "ignited" still, how is anyone in the Middle Ages going to control the time - or place - at which the lava is released? Failing a big nuke, doesn't that, back then, have to be by magic? $\endgroup$ Nov 17, 2021 at 20:50

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