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Army 1(the defending army) has the city of Phoenix, Arizona.

Army 2(the attacking army) has an underground tunnel under the city. The tunnel circles the city under the outskirts, has a few spokes (between 4-10, haven't decided yet)going towards the center where they plan to spring from the ground and attack Army 1, and one long tunnel that goes back to the entry point. So, wagon wheel on a stick.

Army 1 has learned of this attack, and needs to stop it. Army 1 also has a high ranking Army 2 official willing to work with them(as long as the plan isn't blatant suicide). Army 2 thinks this officer was captured, and if he appears in the tunnel after 'escaping' he can try to do something.

Limits;

  • Army 2 officer/Army 1 spy cannot die(or at least thinks he won't).
  • Assume modern tech, although it is slightly in the future but weapons are mostly the same.
  • Phoenix needs to not collapse in the process(I think that rules out big explosives).
  • The tunnel is too big to gas.
  • Solution preferably has some kind of explosion for a fun story.
  • Army 1 cannot enter the tunnel without death happening to them(unless the fortifications were somehow neutralized)
  • Two days to prepare, then three hours to finish plan.
  • If Army 1 just blockades the exit, Army 2 will just come up somewhere else. If Army 1 keeps blocking exits, Army 2 will eventually just pack the tunnel with explosives and blow up the city.
  • Minimal loss of Army 1 life
  • More upon request

My first idea was to gas the place but that's not interesting, and big explosives would just bring down the city, and although that would look cool I need the city later. I thought maybe Army 2 officer/Army 1 spy could run through and drop methane into the tunnels, then get out and Army 1 could light a match and use up all the oxygen/burn the soldiers, but idk if that would bring down the city. Any suggestions?

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    $\begingroup$ What kind of tunnel? Subways? Underground freeway? Ye olde mineshaft? $\endgroup$
    – Ryan_L
    Nov 16 '20 at 21:42
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    $\begingroup$ The tunnel most certainly is not too large to gas. If they can dig it in a reasonable time and have a reasonable chance it does not collapse on their heads, it's not too big to gas. Nerve gas can be lethal at the part-per-billion range. Even hydrogen-sulfide (rotten egg gas) can be lethal at 200 ppm. Carbon monoxide can be lethal at 200 ppm. So a cubic meter of CO can fill 5000 cubic meters of tunnel to lethal level. $\endgroup$
    – puppetsock
    Nov 16 '20 at 23:03
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    $\begingroup$ What did they use to dig the tunnel? Spades? Tunneling machines? If machines the tunnel should probably be fairly deep down to minimise the likelihood that the vibrations were felt. Also means the chance the buildings above are less likely to collapse if there is an explosion. If spades we can assume the tunnel isn't very large and so there is a fairly safe assumption you can fill it with gas. $\endgroup$ Nov 17 '20 at 13:40
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    $\begingroup$ "The tunnel is too big to gas." What is this statement supposed to mean? It it QUITE possible to gas the whole city on the surface, which is exposed to the near infinite atmosphere, using ONE gas bomb. How are the tunnels supposed to be "to big"? (insert 743 "?"'s here,please)... this is a ludicrous limitation. Rather say something like gas forbidden by war conventions, or something. $\endgroup$
    – user79911
    Nov 17 '20 at 14:40
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    $\begingroup$ A tunnel, dug ALL THE WAY AROUND Phoenix, stealthily? Solution: Hire the entire invading army as construction workers, they outperform our own diggers by a factor of 50 $\endgroup$
    – user79911
    Nov 17 '20 at 14:58

18 Answers 18

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Some options

  • Flood the tunnel from a river, reservoir, etc. [EDIT] Come to think of it, redirecting rain sewers or even actual black-water sewage (i.e. human excreta https://en.wikipedia.org/wiki/Blackwater_(waste)) into the tunnels is another option.

  • Bulk-pumping gasoline / diesel into known exits to turn the tunnel into a barbeque. [EDIT] Even simply burning off all the oxygen using combustion would render the tunnel uninhabitable; military CBRN gear (gas masks), I believe, are filters; they do not have an oxygen supply. [EDIT #2] CBRN gear might not protect against gasoline fumes either.

  • Personally, I don't see any problem with thermobaric / fuel-air explosives or even natural gas to create an explosion. The blast will be channeled along the tunnels and so negligible damage should occur to buildings (remember they are designed to withstand earthquakes).

[EDIT #2] Oh, how could I forget, there's all sorts of other fun things available in a major city. According to Wikipedia: "Manufacturing now ranks third among Phoenix's industries, and includes the production of ... chemicals, and processed foods."

  • A real world example of death by innundation in foodstuff is the https://en.wikipedia.org/wiki/Great_Molasses_Flood but you could substitute soda, beer, or other liquid mass produced foods.

  • The defenders would also have access to the chemical plants and chemical tanker cars from the rail depot that supply them. Of course, the plants also are staffed with a handy crew of chemical engineers to do something creatively unpleasant with them to send flowing through a tunnel. All sorts of acids or other toxic compounds might be possible. Military CBRN equipment would probably not provide protection against these.

Combine some or all of the above for extra fun.

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    $\begingroup$ Flooding a giant tunnel requires lots of water and done quickly could undermine the city and cause a collapse. The gasoline/methane isn't bad, as you wouldn't even need to set it on fire to drive everyone out, but I don't know how much you'll need (probably tap into natural gas supply for the city). Big explosion/fire underground may knock out supports and cause collapse/city destruction. $\endgroup$
    – DWKraus
    Nov 16 '20 at 21:47
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    $\begingroup$ @DWKraus Flooding the tunnel would only undermine the city if the defender did nothing to stem the water flow over a period of weeks, if not months. The OP said that the tunnel covers the outskirts and only has a few spokes going into downtown. Presumably, the crisis would be over and the flooding could be terminated well before significant undermining could occur.. $\endgroup$ Nov 16 '20 at 23:55
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    $\begingroup$ I like the sewage. Nice addition. $\endgroup$
    – DWKraus
    Nov 17 '20 at 1:38
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    $\begingroup$ In Phoenix, unless this is taking place during a summer monsoon, there's not likely to be much happening in the rain sewers, rain-wise. $\endgroup$
    – AndyDan
    Nov 17 '20 at 15:24
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    $\begingroup$ How can the tunnel be too big for gas, but not be too big to flood? $\endgroup$
    – D.J. Klomp
    Nov 18 '20 at 15:02
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Smoke

It's the bane of fire fighters everywhere. It blocks vision and chokes anyone without breathing gear. enter image description here

Setting fire to a vehicle would fill the area quickly with smoke forcing everyone out. Anyone not fast enough would choke to death.

It's an easy, quick method and doesn't require any gear.

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    $\begingroup$ For bonus harm, burn a bunch of old tires. $\endgroup$
    – Nelson
    Nov 17 '20 at 6:23
  • $\begingroup$ Ignoring that the OP said no gassing, Phoenix is roughly 33 miles in diameter and, according to the OP, there is a tunnel that runs the circumference, an estimated 207 mi. length). You'd need to burn 17 vehicles or more per mile of tunnel, without assaulting your way down the tunnel, to deny the entirety of tunnel to the attacker to avoid the "pack the tunnel with explosives and blow up the city" scenario. Seems improbable. $\endgroup$ Nov 17 '20 at 10:11
  • $\begingroup$ +1 for smoke! I didn't think of that one! The output of manufacturing plants could be redirected into the tunnel. I don't know if there are any coal-fired electrical plants near Phoenix, but there probably are. Redirect that into the tunnels and scream "whadaya think of our carbon footprint!?" $\endgroup$ Nov 17 '20 at 14:28
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    $\begingroup$ @GrumpyYoungMan Why 17 vehicles specifically? Depending on the diameter of the tunnel, it seems very high to me... $\endgroup$
    – JS Lavertu
    Nov 17 '20 at 20:52
  • $\begingroup$ @JSLavertu The original question says the tunnel is too large to be easily gassed so a basic estimate is 1 burning vehicle for every 100 yards of tunnel equaling 17 vehicles per mile. Couple that with the fact that the attacking troops control all 207 miles of tunnel and won't let the defender drive 3,519 vehicles in without a fierce battle means this plan can't be implemented. $\endgroup$ Nov 17 '20 at 21:15
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These attackers are total boneheads. Let them come!

You have an attacking force coming. You know exactly where they will come - a choke point where they must emerge from the tunnel. Once they are out you can prevent them moving farther into the city and you can prevent them from leaving the way they came. These guys are boneheads! Let them come!

You know where they will come out of the ground. Fortify the area around that. Do it in a way not immediately obvious from the tunnel mouth. They will come out of the ground thinking they are stealthy ninjas. They will quickly realize that away from the immediate exit from the tunnel, they are surrounded by collapsed buildings, walls, barbed wire and gun emplacements.

They will want to turn around and go back the way they came. But no. You collapse the tunnel behind them or fill it with rubble. Ideally this occurs quietly, about 500 meters behind them. Then when they retreat from the untenable situation topside they find themselves in a short dead end tunnel.

You will accept their surrender. If they are disinclined to surrender they will become more inclined when they get thirsty.

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    $\begingroup$ I have to agree with this. The sheer mathematics of getting 2-3 soldiers at time out of even a middling wide tunnel makes tunnelling unlikely to succeed as anything but a distraction from another attack front if discovered early enough. The "stick" of the wagonwheel is another major chokepoint. If discovered later - how do you hide all the soliders that came out of the tunnel. I would go back to the drawing board tbh. $\endgroup$ Nov 18 '20 at 9:48
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    $\begingroup$ This is really the only valid answer tbh. If the defenders know about the tunnel, it's not an attack tunnel, its a ready made dungeon that your entire opposing army is cheerfully walking into. Still think you should fill it with bees though, just because any army that stupid needs several million bees to help them realise what boneheads they are. $\endgroup$ Nov 18 '20 at 12:09
  • $\begingroup$ It's not a valid answer because the conditions of the question already says it isn't: "If Army 1 just blockades the exit, Army 2 will just come up somewhere else. If Army 1 keeps blocking exits, Army 2 will eventually just pack the tunnel with explosives and blow up the city." $\endgroup$ Nov 19 '20 at 3:56
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    $\begingroup$ @GrumpyYoungMan You don't blockade exit, you are aware of its location and make traps of nearby, but not close surroundings. If the attacking army can learn of you being aware of all tunnel exits, the original question doesn't make sense, as the only way to block tunnels is to fill them up with rubble. Everything else is just temporary. $\endgroup$
    – Gnudiff
    Nov 19 '20 at 9:19
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Murder Hornets

I have no idea if this is practical, but just because no one has mentioned it and it has a certain Rule of Cool cachet - swarms of murder hornets.

Also stick a few hundred thousand Africanised honeybees and maybe some angry army / bullet ants. Just for luck.

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Dirty bomb:

No tunnel is too big to gas. You can gas whole cities. But no gas. Phosgene oxime (not technically a gas, but often aerosolized) would even coat the surfaces in powerful blistering agent, making the tunnel virtually unusable. Nerve agents in tiny parts per billion would make the air unbreathable as they evaporated and can be absorbed via skin over time. They often are contact poisons that really affect those who touch them. Shellfish toxin is nasty and a vial you can hold in your hand will kill a whole city. So chemical weapons but not gas.

I'm a biology guy, but good suits will protect you, and the tunnel can be sterilized. Anthrax spores will render the place pretty unuseable, but have a nasty tendency to spread a lot, since infection spreads (especially near a civilian population center) and they track on shoes, etc. Anthrax spores are REALLY hard to get rid of, though, so the tunnel is likely unuseable. But generally biological weapons spread over time and your enemy has to know and believe in them to be deterred.

My best thought is radiation. You can try a dirty bomb (non-nuclear radiological) where the radioactivity means people don't die right away, but require extensive medical treatment (debilitating to the enemy ability to fight war). The tunnel is filled with lethal radioactives that render it uninhabitable. An explosion scatters the stuff around very efficiently. Long term, this can spread into the water table, but war is full of desperate compromises. Your enemy will likely have radiation detection equipment, and if not will rapidly discover the attack as soldiers quickly get sick. Radiation is easily verifiable with equipment unlike biologicals.

Finally, how about area denial munitions? It's a fancy way of saying tiny anti-personnel mines. They are usually deployed from aircraft, it would be tricky to get missiles launching tiny mine bomblets underground, but once you did, the large but relatively small cave would become a giant booby trap. People moving around would set off explosives, and they're all small (designed to kill/maim individuals) so they won't "bring down the house." The weapons are already in the USAF arsenal, so you wouldn't need to make something new. Bigger munitions will blow up vehicles as well, so if the invaders have "tunnel tanks" and earthmoving equipment, it can be destroyed as well.

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    $\begingroup$ Depends on whether the attacking army is an actual well equipped modern military with CBRN gear or not (e.g. en.wikipedia.org/wiki/MOPP_(protective_gear) ). The CBRN gear will protect its wearer against common military toxins and bioweapons. $\endgroup$ Nov 17 '20 at 0:20
  • $\begingroup$ @GrumpyYoungMan I agree, which is why I gave a range of answers. Long-term, a tunnel with NBC contamination is pretty uninhabitable. I work in a hospital, and good working all day digging in full-body PPE. COVID is bad enough. Also why my favorite was a radiological weapon. $\endgroup$
    – DWKraus
    Nov 17 '20 at 1:35
  • $\begingroup$ Also for radioactivity. Its a perfect solution. We just need an element with a quite short half life (couple of days to 5 years maybe), and the collateral damage would be minimal. $\endgroup$
    – Hobbamok
    Nov 17 '20 at 9:02
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    $\begingroup$ @Hobbamok While I like the idea of radioactives, Palo Verde Generating Station (the nearest nuclear reactor) is 45 miles from Phoenix. The attacker would have access to nuclear waste but not the defender and I'm hard pressed to think of anything else that would provide the levels of radioactivity needed in the amounts needed to deny the entirety of the tunnel to the attacker. $\endgroup$ Nov 17 '20 at 10:22
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Tunnels are by nature a scary environment. Psychotropic substances in the air, deployed by fast flying drones could render the soldiers below suspectable to fighting among themselves.

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  • $\begingroup$ Or maybe they would drop their guns and walk off into the desert looking for God. $\endgroup$ Nov 18 '20 at 0:00
  • $\begingroup$ @JoannaMarietti makes a good point, but it's more likely they'd sit down and enjoy the show. Drug houses exist because addicts tend not to move around when in the throes of an hallucination. But I would hate to meet a whole bunch of dead-heads armed to the teeth. Yikes! $\endgroup$ Nov 18 '20 at 5:12
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    $\begingroup$ 2 issues. 1. How is a pychotropic substance not a gas, that the OP has already discounted? 2. A flying drone is literally a very poor fit for a tunnel - coz a) walls b) line of sight comms. Drones are also fragile and easy to spot and shoot down. It would be easier than shooting womp rats in beggars canyon. $\endgroup$ Nov 18 '20 at 9:53
  • $\begingroup$ 1) Lots of things that float in mid air are not gasses, but little droplets aka aerosols(fog) or particles (dust). If the distribution is a problem, maybe use a pre-existing system (fire sprinklers) or halon-fire-extinguisher. Or let the enemy set it off himself, when they breach for the surface. $\endgroup$
    – Pica
    Nov 22 '20 at 19:48
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Assumption: The tunnel is very well reinforced, which is the only way you can have an explosion (bullet #5) and not collapse Phoenix (bullet #3, at least not those parts of the city over the tunnel).

Assumption: The tunnel as not been breached on the city-side. In other words, Army #1 has control over their "side" of the tunnel (they can choose where to pierce into the tunnel.

Assumption: A city the size of Phoenix will have ample amounts of the materials suggested. (May not actually be true....)

  • Halon: Used in fire-suppression systems to rob an area of oxygen without soaking everything down with water. Can be injected anywhere, but near the center would be better. Just keep on injecting until you see people passing out/dying at the entrance. Would make the tunnel unusable for days — probably weeks.

  • Chase Army #2 away from the entrance and start a fire at the entrance. It'll take a bit, but that'll suck out all the oxygen, too.

  • Propane: Liquid propane expands at the rate of 10,000:1. That means a gallon of propane makes 10,000 gallons by volume of ignitable gas. (I haven't verified that number, it was what Utah State claimed in their training manuals for licensing to dispense liquid propane. I'm just rolling with it.) A little math to estimate the amount of you need... fill tunnel... add match. The resulting "boom!" coming out of the open end of the tunnel might be heard in Kansas.

  • Sewage: Tell me Phoenix doesn't have enough of this. ("Alright everybody, we need an emergency blow! Eat all the chili you can and flush all at the same time in an hour!") You'd want to push this in at several points. Might not stop Army #2... but they'll wish they were anywhere else while they attacked.

  • Pump all the 2-chlorobenzalmalononitrile you can into the tunnel. Better known as "Tear Gas," It'll disable Army #2's efforts for as long as you can keep pumping the gas.

It's worth noting, after saying all that I have, that you could just have the city's various industries divert their usual dumping of toxic industrial waste in the local rivers into the tunnel. Green Peace will freak out, but the population's option is to endure slavery under the iron boot of Army #2, so I doubt you'll hear too many complainers.

BTW #1, yes, I ignored your limitation on gas. This is the modern world. Gas of many kinds are not only easy to make, but are in plentiful supply. Remember that 10,000:1 expansion rate.

BTW #2, I didn't suggest gasoline because, being a liquid, it's expansion into gaseous form would be too slow. When you lit the match, you'd blow the city sky high. There'd be too much of it near the injection points.

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    $\begingroup$ Yep. OP banne gassing the tunnel, but no, thats the solution. If the tunnels actually are too large to be gassed Army #1 should just surrender already because army #2 has basically built a new world wonder under their feat and with that engineering can probably do whatever they want $\endgroup$
    – Hobbamok
    Nov 17 '20 at 9:05
  • $\begingroup$ Fill the tunnel with propane, drop a match and nothing happens--it's not an explosive mix. $\endgroup$ Nov 18 '20 at 4:56
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    $\begingroup$ @LorenPechtel Propane without oxygen doesn't burn. Yup. No combustible will burn without oxygen. That's why my answer talks about the 10,000:1 expansion ratio. That expansion is the conversion of liquid propane to gaseous form mixed with oxygen (1 liquid gallon of propane creates 10,000 gallons by volume of combustible gas: propane + oxygen, which I assume the tunnel is filled with). I didn't think I had to actually explain that to this level of detail. $\endgroup$ Nov 18 '20 at 5:08
  • $\begingroup$ @JBH You seemed to be describing filling the tunnel with propane. Actually, even if you get the ratio right it's not going to work too well because the explosive range is low and there's no way you'll get the mix even. $\endgroup$ Nov 18 '20 at 5:32
  • $\begingroup$ @LorenPechtel No offense, but I'm going to ignore that last comment until you've taken the time to go watch some YouTube videos about the explosive nature of propane. Then, you need to go do some research about mine gas explosions, which are only due to methane, not the more combustible propane. $\endgroup$ Nov 18 '20 at 6:27
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Remove all the oxygen from the tunnels.

Do this by stockpiling large quantities of liquid nitrogen ahead of time (just requires energy to extract from air - which is mostly nitrogen). At the time of the attack, pour liquid nitrogen into available openings. The cold will kill anyone too close to an opening. As the liquid evaporates the gaseous nitrogen will displace all the oxygen from the tunnels.

You could also use liquid carbon-dioxide. Might get that as well when collecting liquid nitrogen from air, as a side-bonus.

The in-tunnel army will become incapacitated and die within a few minutes, due to lack of oxygen.

The above-ground damage and causalities will be minimal. You will want to evacuate the above-ground civilians ahead of time. The cloud of gas (really a lack of oxygen) will dissipate in a few days (or less if there is a good wind).

No fuss, no muss - though lacking in drama.

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Cheating slightly - I'd suggest liquid nitrogen (shortly to be gas)

A large city should have plenty - it's a common industrial and research coolant

It is also heavier than air once it evaporates (because it's colder), tending to collect in hollows

If your spy can alter the tunnel plans to have a slight dip, with ideally a curve in the tunnel at the bottom the nitrogen could be pumped in, as a liquid, filling this dip with a lethal concentration. Soldiers would walk in, then die. The curve might stop soldiers at the back noticing that their friends had dropped dead.

A high concentration of nitrogen will cause fainting, then death, due to lack of oxygen. It's worth noting that ill effects from this won't be felt until soldiers are near losing conciousness - unlike smoke, it doesn't trigger CO2 receptors in the lungs that tell you that the air is bad to breathe.

Slightly further away, soldiers are likely to behave erratically, because of low oxygen. With any luck, a tunnel full of armed men with diminished reasoning capacity, on high alert, would turn into a bloodbath.

The liquid nitrogen can be dispersed through pipes, either that have been cut through by the tunnel already, or by drilling small holes through the tunnel roof. Very little specialist equipment is needed to handle it, it's non toxic (once it disappates) and presents no risk to the city or its inhabitants above. It won't be filtered by standard chemical filtering gear, and would require an air supply to mitigate.

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Hold on a minute, I think you're overthinking this.

The tunnel system follows a wheel-and-spokes arrangement. The invading army uses the spokes to travel from the outskirts of the city into the city center. Let me word that slightly differently. The invading army has loaded themselves into a enclosed tube that's several miles long and only has exits at the extreme ends. That's pretty much the worst situation an army could put itself in. All you really need is a good old-fashioned ambush.

At the far end of the tunnel (where the invaders plan to emerge from), build an armored, camouflaged barricade that you can hide several fast-firing heavy weapons behind (such as M134s, conveniently manufactured by local company Dillon Aero). Don't risk sending gunners into the tunnels, hard-mount the weapons in place and control them from the surface by wire. When the invading army gets close, mow them down. The invaders are trapped with nowhere to go. Your shots can't miss if they're pointed straight down the tunnel, it's shooting fish in a barrel. The invaders can't stop the barrage because there are no gunners to kill with a counterattack, plus most of them can't fire a weapon without hitting one of their own (the space restricts them into a deep formation that's only 2-3 men wide). Their only escape is to run back down several miles of tunnel, with heavy weapons firing at them endlessly.

Additional options for extra fun:

  • Have a team of gunners with similar anti-infantry heavy weapons drop in at the other end of the tunnel. When the attackers start to retreat, your weapons emplacements at both ends start slowly moving to the middle (but never getting close enough to hit each other).
  • Flood the tunnels with water to a depth of 6-12 inches. That's not dangerous on its own but it will slow down a fleeing enemy considerably (which is just as bad when you're trying to flee from bullet hell).
  • Once the invading army enters the "spoke" tunnel but is not yet close enough to ambush, cut off their escape by blockading the "wheel" part of the tunnel on either side of where it meets the "spoke" tunnel. These blockades shouldn't be visible from the "spoke" tunnel, you want the invaders to run all the way back down to the other end before they realize they're trapped.
    • You don't even have to block the tunnel completely. Something as simple as parking a heavy vehicle in the tunnel will create a choke point that delays escape and requires the enemy to squeeze past one at a time. Limiting their rate of retreat makes them easier to attack and limits their ability to harm your own troops.
  • A long tunnel will also channel pressure waves. Disorient the fleeing enemy troops with an acoustic weapon, or detonate explosives (not enough to damage the tunnel) that have high detonation velocities and let the shock waves tear through the enemy lines. Shock waves can cause organ damage and internal bleeding, even with no signs of external injury.
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Spread a disease

I've read a lot of interesting answers, but none of them seem to be taking advantage of your spy.

Incubate him with a deadly or incapacitating virus/bacteria and send him into the tunnel. You could vaccinate him first, or at least make him believe he has been immunized.

He goes in. He is quickly received by their comrades who hug him and shake his hand congratulating him for his heroic escape. Two days later a harsh fever or disenteria strikes in, incapacitating ~80% of the enemy forces. Army 1 attacks the tunnel.

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Natural Gas Odorants

As natural gas does not have its own odor, mercaptans and other chemicals are often added to give it a smell. This helps consumers detect a leak in a natural gas line. Simply spray some of the odorant at the entrances and ventilation ducts to the tunnel system. This will make the occupants suspect a gas leak -- even though no one actually occurs -- and they will abandon the tunnels for their own safety.

  • No actual natural gas needs to be used.
  • No actual risk to people or property. The effect is entirely psychological.
  • You don't need to fill the entire tunnel system, just key points such as entrances and ventilation ducts.
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Elephant's Toothpaste

You wanted an explosion. Here's your explosion.

Stockpile large amounts of hydrogen peroxide and potassium iodide. When the time is right, seal the city center entrance and pump both chemicals into the tunnels with industrial pumps. When the two chemicals combine, you get a violent exothermic reaction.

It's not hot enough to hurt anyone, but being surrounded in foam is going to severely inhibit mobility and suffocate anyone without proper breathing equipment. This is enough to give the surface army a significant advantage.

Mixing in a strong acid or base would additionally cause chemical burns and destruction of equipment. Hydrofluoric acid is a good choice for dissolving guns and other metallic equipment. Something like sodium hydroxide would be better for dissolving organic material such as clothing or flesh.

(someone with a better understanding of chemistry can advise if introducing a strong acid or base will interfere with the rapid decomposition of hydrogen peroxide catalyzed by potassium iodide)

AB Foam would have a similar effect if you wanted to encase the enemy army in polyurethane foam.

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A tunnel as big as you describe needs a ventilation system to pull air in from above and eliminate carbon dioxide, not mention any exhaust from vehicles or machines. Find the ventilation system and disable it. This will likely have the added effect of making the environment uncomfortably hot. No one is going to be having any fun down there.

This will almost certainly take more than three hours to start affecting the underground army, but the rule is "three hours to finish plan," not "three hours to take effect."

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We can't gas them with toxic or nerve or oxygen-depriving gases because they all have magic RebreatherTech (tm) and are immune to gases.

If you ignite a gas in there so that it explodes with enough force to do any significant harm to the ungassable inhabitants, you'll probably blow up great swathes of the city.

They can dig crazy fast and have magic breathing apparatus, so this is sci-fi, so we can make up tech.

That gives a couple of ideas.

Option one is just to redirect a river in there. Makes for a nice scene underground as first there's a breeze in the tunnel, then the roar of the water is heard, then a frothing wall of death coming along the tunnel like an express train.

But that might be too obvious. Plus, they can have some sci-fi bulkhead doors in the tunnel to isolate the flooded section.

Option two is to trickle in a liquid or dust very slowly, through lots of small holes, until all sections of the tunnels are a little bit covered with it.

Tunnels should have drainage for natural water leakage, so this should go unnoticed until all the drains have some in, along the whole length.

After a certain period, microcapsules of reactant in the water dissolve and release the reactant, that very quickly converts the first liquid into a rapidly-hardening foam that expands and fills the tunnels, trapping everyone within it. The city's security force can then enter the tunnels, dissolving the foam as they go, exposing the attackers one by one where they can be easily captured and handled.

Now the city has a large number of hostages, and control of the tunnels.

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I find it hard to belive that a Limited Underground Volume is to big for Gas. If we assume Modern ish tech, it should be no problem to map the Tunnels. Figure out where the Access Points, how deep it is and so on. With that intel, putting gas into the Place is the best way. The Gas will only kill the people and not blow the Place up. Plus you can fill the Place with Gas nobody can detect.

But ok, no Gas.

You could try to use a Biological Weapon. A Virus or similar that spreads and kills fast. This could be injected via either the Ventelation System or Water Supply.

Another idea would be a bit Sci Fi to say the least but you could try to melt the Place. Or Microwave it i suppose. It is a bit out there but in principle, Microwaves can go through the Ground. But the Generator would have to be quiet big to say the least.

Imo, the Virus idea is probably the best one if you dont want to use Gas.

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  • $\begingroup$ I agree about the gas: I suggested alternate chemical weapons (contact not gas) as a side-step. $\endgroup$
    – DWKraus
    Nov 16 '20 at 21:49
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I'm not figuring out how to get Google to cough up the name of the chemical, but it's commonly used in fuel-air bombs. It's a gas that can be liquefied by pressure, the important characteristic is a very wide explosive range.

Drill down almost to the tunnel in several spots, place an explosive charge and above ground prepare to quickly pump the stuff in but don't cover the holes just yet.

Fire the charges, making holes into the tunnel, pump in gas, detonate.

While the total boom won't be all that big the confined space will make the shockwaves very nasty for the troops inside and it's going to burn up a lot of the oxygen.

As soon as the blast wave clears you repeat the process. If you get rid of a bit over half the oxygen the troops inside suffocate. They're going to be busy donning chem warfare gear when the gas pours in, it's going to do absolutely nothing because the threat isn't toxins, but hypoxia.

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M18 Claymore Mines would be a good start. After that, Do the defenders of the town still have access to either Phoenix International or Luke AFB over in Glendale? Depending on the depth of the tunnel (thinking it'd be fairly deep if it's large enough to negate a gas attack) your defenders could use jury-rigged munitions from Luke, not sure if the KC-135R's over at KPHX would be any use, but that is a lot of JP8 fuel to pump down the tunnels. Won't burn easily, but it will burn hot.

You mentioned fortifications, are those protecting the single entrance/exit (if I'm picturing this right) or the attack point up into the city?

Also, I didn't see a timeline mentioned, but if you've got access to either airfield then you've also got access to every other still functioning military base in the CONUS. We do have bunker busters in the inventory that are meant to penetrate protected structures underground and collapse them with minimal damage to the surface. The GBU-24 would probably do it, but there would be some collateral damage to buildings and roads.

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