# What do my wizards drop on towns to turn them into craters? [closed]

During excavations some wizards came upon the workshop of the 5th dimensional being that created everything, essentially god. In that workshop were many notebooks chocked full of the techniques of creation (Turning 5th dimensional magic energy into 3 dimensional objects) Every once in a while godly existence gets boring and you need to spice things up, so some notebooks contained various ways of destroying things. Over a few hundred years our intrepid wizards translated one such way. It involves dropping something. Luckily wizards have access to airships with a ceiling of 5000m.

Rules:

1. It can be any currently known natural substance or substances in our world even if the wizards have no idea what it is. Natural is key. Avoid Man-made substances/chemicals.
2. The least complex the better.
3. No nuclear bombs, balls of pure gunpowder, or metal spheres filled with nitroglycerine. (See #1 on man-made)
4. It must fall a minimum of 300meters and a maximum of 5000.
5. Weight limit: Don't drop a mountain on the town.
6. The wizards should be safe from the resulting destruction. (Most of the time. Some of the larger craters will have definitely destroyed the wizards who made it.)
7. It must be scalable. The God-creator used it to make a 800km diameter crater. The wizards will use it to make craters between 100m and 15km.
8. When the wizards are “creating” the object it will be stationary and inert. Once their spell is complete it will become “fully real” and any chemical reactions will begin.
9. The wizards need to be within 30meters of the object while creating it. So when it is “fully realized” the object must not instantly kill them by its proximity.
10. The object created is permanent and must not cause undue devastation to the world beyond its initial impact.

Notes:The object doesn’t necessarily need to be explosive, but I feel that’s the only way. I’ve done the math for the impact of a 100 ton pure iron sphere dropped from 5000m, but it doesn’t have nearly enough energy for what I’m looking for. I originally wanted the objects to be actual meteors falling from outer space or teleported into the atmosphere, but my magic system doesn’t really allow for that. There have been questions about medieval explosives, stone age explosive, and others, but I believe this question to be unique and not a duplicate due to the criteria.

• Too many conflicting/broad requirements and the hard science tag make this unanswerable. 15 km is in the range of Chicxulub crater, just to say one, and you both want something natural but explosive...
– L.Dutch
Apr 19 at 13:23
• A blast or explosion of such magnitude would seem to certainly destroy the fragile airship only 5000m away. Either by the hammerblow of the shock wave, or by the shredding action of the voluminous non-molten airborne debris, or by incineration from the voluminous molten airborne debris, etc. Apr 19 at 16:39
• Okay, so you said explicitly "no nukes", but some, as I understand it, are little more than enriched fissile material (which could be somewhat argued to be "natural") and a system to bring two pieces together when you want them to go 'boom' (which, for an impact-detonation, could be pretty simple). The only thing particularly "artificial" about ²³⁹Pu is that it isn't normally found in concentrated form. Apr 19 at 18:03
• @Matthew "The only thing particularly "artificial" about ²³⁹Pu is that it isn't normally found in concentrated form." Well, that and the fact that it's not found at all on planets old enough to have evolved life (at least as we now understand that process). Very rare element in supernova remnants, and then it has a short half life relative to the length of time from the explosion that scatters the star core until the planet it winds up in has even formed. That's what "not naturally occurring" means in terms of radionuclides. Apr 19 at 18:22
• Can I ask why such specific limitations? If this is coming from texts developed by your stories "god" then why the limits? And why couldn't they just send meteors from space? Works IRL.
– Len
Apr 19 at 19:09

Ever tossed a river rock into a campfire? Probably not -- camping seems to be restricted to those who are fanatical about it, these days, instead of being forced on clueless children.

I mention this because a waterlogged stone will explode in a fire when it gets hot enough for the water trapped in the rock to flash into steam. People have been injured by this phenomenon with rocks the size of a fist.

What your wizards need, then, is a source of heat that they can put inside a large, waterlogged stone (ideally one shaped so that when it drops, it'll penetrate the ground and spend its energy moving the earth rather than just spraying fragments of itself). Fortunately, any mixture of sodium, potassium, and lithium will do this, and provide a certain amount of delay to allow for the "bomb" to fall. If the wizards can't create alkali metals, they could accomplish much the same effect with sodium or potassium hydroxide (which will get very, very hot when it reacts with water in any form).

Now, it would take a god to make a crater 800 km across this way -- but if you're willing to handwave that, the BLEVE type explosions these "steam rocks" can create might well produce shallow craters as large as several hundred meters across, depending strongly on the soil they're dropped into.

• This is also the same mechanism that causes boiler explosions. Some of the worse boiler explosions tore whole ships apart and killed thousands. Adding in the sodium/potassium is brilliant for building pressure rapidly. Apr 20 at 0:03
• This is the best answer so far. As an Eagle Scout I may or may not have done this a couple times in the past... Apr 20 at 1:08
• Brilliant. I didn't even think about chemistry! Strong acids will also give of a lot of heat in water, so even more options. Apr 20 at 1:16
• Right! Igneous rocks for campfires, not sedimentary! However, you still need to provide enough energy to force a phase change. The rocks are not explosive, they just hold water, boiling the water explodes the rock. It would be great for creating massive sinkholes if they victims lived on or near sedimentary rocks. That would create the crater effect. You still need a ton of energy, but you don't need to deliver it in a microsecond. Apr 20 at 4:56
• @TheDaleks As a keen camper from an early age, I haven't done this, precisely because I've been warned about it. Some things are "that's cool, got to try it", like Roman candle fights (with gloves, because even crazy people can feel the tube getting hot). Exploding rock shrapnel taking your face off, not so much. Apr 20 at 13:38

# You cannot simply "drop" something.

If you can't drop explosives, you're left with plain ol' impact force. But nothing lighter than a mountain will do (in fact, it must be much heavier).

You have imposed a vague weight limit:

Weight limit: Don't drop a mountain on the town.

Note: I'm taking you at your word and treating this as a limitation on the mass of an object, rather than the size.

You have imposed a specific height limit:

It must fall a minimum of 300meters and a maximum of 5000.

You're asking for a crater about one tenth the size of the Chicxulub impact crater, which is around 180km wide. It is estimated that the total energy of the Chicxulub impact was between $$1.3×10^{24}J$$ and $$5.8×10^{25}J$$.

Now, there are various estimates of the exact relationship between crater diameter and the energy of an impact, but we know it is approximately $$K\approx D^3$$,[1,2,3] so if we reduce the diameter by a factor of 10, the energy is reduced by a factor of 1000. Therefore, we're shooting for at least $$1.3×10^{21}J$$.

Now for why what you ask is impossible: A height limit plus a weight limit imposes an upper bound on the energy of anything propelled by gravity.

Specifically, $$E = \frac{1}{2}mv^2$$, where $$m$$ is the mass of your object, and $$v$$ is the final speed of your object. What will be your final speed? With your wizards' magical ability to negate friction around their object, we will neglect air resistance. Now, we could calculate these integrals ourselves, but we're smart internet users so we can use this free fall calculator to find a fall time from 5km of 31.93 seconds, giving a final velocity of 313.16m/s. This is the maximum possible speed given your height restrictions and allowing zero air resistance.

Plugging this in to our kinetic energy expression, we can see that for the energy we want, our mass would need to be around $$1.3\times 10^{16}kg$$. Is that heavier than a mountain? The tallest mountain (from sea level) is Mt Everest. A nice back-of-envelope estimate at https://weightofstuff.com/how-much-does-mount-everest-weigh/ gives a whopping 357 trillion pounds, which is $$1.62\times 10^{14}kg$$.

Even if we stretch the limits you gave to the extreme, we're still a hundred times too light for the crater you want.

## So, what can we do instead?

We've ruled out just dropping a thing. The size limit and speed limit make it impossible. You've ruled out making the thing heavier, so all that's left is to make it faster. Don't drop it at the ground, throw it! Take a 2-ton ball of steel and use your wizardly powers to impose a strong magnetic field to push it down to the ground. With arbitrary amounts of magical energy, your wizard railgun can make that ball go arbitrarily fast and pump as much energy into the crater as you want!

References:

1. https://www.noao.edu/jagi/sepo/education/plansurf/plansurfiiia.html
2. J. W. Bond, The moon and the planets volume 26, pages317–321(1982). DOI: 10.1007/BF00928014
3. D. W. Hughes, Mon. Not. R. Astron. Soc.338,999–1003 (2003).
• You followed my exact thought process on the whole mess. Ive written myself into a worldbuilding corner and came here to see what the brilliant folks here can come up before I go off and scrap some stuff from my world. Putting kinetic energy into objects seems like it eould fundamentally change a medieval world. Imagine the sieges. Apr 20 at 0:42
• If you're open to some more exotic materials, supposing your mages can observe effects of a neutron star and "realize what it's made of" i.e. magically replicate it, you can definitely make something way heavier than rock... Apr 20 at 0:50
• @Hippeus_Lancer, one of the most important factors with using magic in fiction is to keep it under control. The effect you want is that it's barely more effective than the technology of the period you're set in, probably for greater effort. Great works of magic were done long ago by ancient wizards using lost methods or are referred to obliquely, but never actually done. Otherwise it takes over the whole story. Apr 20 at 7:58
• R. Barrett If you could somehow transport a chunk of neutron star stuff away from the parent body, it'd be incredibly explosive on its own, no momentum needed. Apr 20 at 8:13
• @separatrix Oh definitely. These craters were made some 3 ages ago, or about 12000 years. Im just trying to figure out if there is anything significant about them. Say:If I drop an exotic material then there might be mines, or if I drop sodium then it is barren. This is just another layer to my campaign setting to make it "feel" like it has been lived in by actual people. Apr 20 at 8:17

## Sodium

Sodium is a basic element that can be using for making bullets that explode violently on impact https://www.youtube.com/watch?v=T85d7ST2yxU. Because it is a pure element, it's composition could not be any simpler. I can't find any exact calculations, but if a simple handgun bullet can contain enough sodium to make things explode this dramatically, I'd imagine dropping a 100 ton ball of the stuff would produce a very impressive crater.

Apart from being highly explosive, this solution is good because the fallout would quickly react with various elements in the ground forming common salts. While your target zone may be rendered agriculturally ruined for a few decades, it would not be so toxic as to be a threat to people settling the area thereafter.

• Would love to see some estimate of how badly you have ruined the soil here. Nothing will live there for a long time, and I would bet the salt runoff would spread the area even futher... Apr 20 at 1:06
• Yes, the soil won't be great for farming, but this method will provide enormous resources for alchemy, chemistry, manufacturing, the lot. Table salt, baking and caustic soda, add water and you get hydrogen gas... what a bonanza! We apparently only make 100k tons/year of metallic sodium, the survivors descendants will be resource rich, and I suppose, quite resentful. en.wikipedia.org/wiki/Sodium#Chemistry Apr 20 at 4:47
• What @chiggsy says, yes, the town would be dead in regards to farming, but even in medieval times you had settlements who could afford to import almost all of their food. One type of settlements that could to that were: Salt mining towns.... Apr 20 at 9:32

I think a microscopic primordial black hole could more or less fit the criteria you give.

Points in favour:

• Certainly not man made (we can't make them!),
• explosion by black hole evaporation can be precisely timed by setting the initial mass,
• all of the intial mass is converted to Hawking radiation in the process, which gives a very significant release of energy even for the most modestly sized of black holes,
• a sufficiently low mass black hole will have a subatomic sized Schwarzschild radius and negligible gravity over macroscopic distances, so before detonation, being within 30 metres of it should not be a problem,
• and the amount of energy released can also be precisely controlled by setting initial mass, so we do have some degree of scalability!

Now, given the size and density of the created object we can safely ignore any interaction with surrounding matter, so after creation our black hole will effectively enter free fall until evaporation. Unfortunately, this means that the time to fall to a safe distance severely constrains our black hole mass; for instance, for a 20 MT explosion, we would want a roughly 1 kg microscopic black hole, which if I have not miscalculated is expected to evaporate in about $$10^{-17}$$ seconds.

However, again given the size and density of the object, impact with the ground should not change its trajectory or development more than its previous impact with the atmosphere, so I think we can simply increase fall distance to achieve the desired level of destruction.

For instance, for a 1000 ton black hole, time to evaporation will be about 84 seconds, allowing for a fall down to 30 km below the surface. During the fall, the black hole will convert all of its mass into energy, for a total energy release of roughly 20 teratons of TNT. The vast majority of this energy release will happen at the very end, i.e. far underground, with the total power output being about double the that of the 2004 Indian Ocean Earthquake (so quite survivable on a planetary scale, but very destructive for anyone right above the hypocenter).

Again, due to the size of the black hole in question, I assume that accretion of surrounding matter is negligible compared to mass loss by Hawking radiation. One should probably also calculate the luminosity at creation. I have treated this as negligible, but have not checked this. Finally, I suppose quantum gravity effects could be important for this kind of thing, so your final mileage may literally vary; but in terms of creating large, scalable explosions from a class of objects that (maybe) occur naturally, it is not a bad fit to your question.

• Dang, I was going to do a write up about this, but you've answered it much better than I can. One thing I can add is that kurzegsagt has a great bite-sized video on this topic. Apr 20 at 14:59

Okay, so your mages know how to access the five dimensions of space and time. This makes things very interesting because one of the cool things about 5 dimensions is that it can hold more stuff. Think about it this way if you have a desk and you cover every square inch of it with papers your desk can't hold any more papers. But you can go in a third dimension and start stacking papers. Now your desk can hold a lot more papers. In fact, if your desk was square, and we imposed a limit that said that you could stack papers until you had a perfect cube, your desk would hold a lot of paper.

Now, then, let's suppose that you have a 4-dimensional sphere move through our 3-dimensional world. What happens? The sphere starts out as a tiny dot, grows to a maximum size, and then it starts to shrink. Just like a sphere passing through a 2-dimensional plane will create a point, then a circle, that gets bigger and bigger, reach a maximum and recede to a point and then to nothing.

Your mages simply have to drop 5-dimensional spheres of iron or rock on these towns. The mage, holds these little pellets of iron. They look like little ball bearings, weigh next to nothing. Give it the old incantation and toss it off the side of the airship. The pellet starts to grow and gain mass, not because ye olde wizard cast a growing spell but because the cross section of the item in our plane of existence is getting larger. It smashes into the town like the wizard dropped an entire mountain on it, but then the object continues to move through the 5th dimensions until it shrinks and eventually disappears leaving nothing but a giant crater where it landed. No exotic materials, no fancy pants explosives, just plain old rocks that happen to be bolders/mountains where most of the stuff/mass is tucked safely away in another dimension that is usually inaccessible to mere humans.

Bonus points, because the wizard didn't check the 5 dimensional size, and the 5 dimensional shape of the object, sometimes as the cross section is passing through our 3-dimensional universe the object randomly and violently generates a giant spike or otherwise juts out in a weird way unexpectedly and damages their air ship. This naturally causes the ship to crash killing everyone on board.

• Alternatively: The object (in our 3D world) does not grow steadily, but very very quickly after a pre-determined amount of time. The rapid expansion would be the explosion. Not the mass of the item itself. Then it would disappear again (as you said). So it stays this small pellet until the trigger. Apr 20 at 9:34
• The object could be geometrically shaped so that the impact with the ground causes it to rotate in the 4th dimension. Apr 20 at 12:17
• @Hobbamok, I actually like the idea of a rapid expansion more than hiding the bulk of the mass in an extra dimension.
– Ryan
Apr 20 at 13:04
• @Ryan technically it is still based on hiding the mass in another dimension. It's just when you "retrieve" that mass and how it causes damage that changes. This way however could even have no "residue" from the impact, aside from some small bits scratched off in the "explosion" Apr 22 at 9:03

Nuclear bomb

I know you said no Nuclear bombs, but it seems a bit narrow. Nuclear bombs are complicated, incredibly difficult to produce and require incredibly accurate science.

But this is not completely true. A lot of science and accuracy goes into enriching Uranium and maximising the yield. In contrast, you've made a scenario in which it's possible to make more rudimentary but highly effective scalable bombs.

Have your mages create a huge cylinder of highly enriched Uranium with a hole in the middle. It's a single naturally occuring element. It'll get close to critical mass. Meaning that if you add a rod of Uranium in the middle it should start fission all on it’s own for a nice boom. Now you only need a way to add the rod automatically after being thrown, but as your mages have airships it seems a small extra step technology wise.

"But won't this much Uranium kill my mages as they produce it?" Uranium is chosen for Nuclear weapons as it's relatively stable over long periods of time, but with a push they can rapidly undergo fission. It makes sense, as otherwise the power would be leaking out of the weapons at a steady rate. With highly enriched Uranium your mages might've got more chance on cancer in the long run, but there isn't an accute danger.

"Won't there be Nuclear devestation afterwards?" Not as much as you might think. The 100% highly enriched Uranium is likely a relatively clean bomb. Most of the radioactive material will only be radioactive in a short duration, expanding their energy. You see people live in the cities already relatively short after the Nuclear bombs fell on Hiroshima and Nagasaki. These bombs are likely more dirty than the ones you'll drop.

• Wouldn't it be possible to build a shape that's stable but goes critical from deforming upon impacting the ground? That makes it safe to handle in the air and leave a crater as it comes down.
– Erik
Apr 20 at 9:09
• @Erik, that was exactly my thought also. Basically, use something brittle to keep the two pieces apart and let gravity bring them together. Apr 20 at 12:30
• These are mages with access to highly advanced magic. They need to make two good-sized balls of pure natural uranium, and place one at each end of the cylinder. Then they make a trigger spell to smash ball A into ball B and (using the 5th dimensional magic) compress the whole thing reeeeally tight. Then they set up a contingency spell (ok, ok, Contingency with Teleport) that will get them the heck out of Dodge when the trigger spell takes effect. Apr 20 at 14:52

Just use air. Really, really highly pressurized air (possibly a pure gas) in a (spherical!) container that is strong enough to contain it, but very brittle, such that a decent impact will cause it to shatter resulting in rapid expansion of the gas inside (i.e. "an explosion").

Your next trick will be figuring out the pressure vessel...

Plasma inside a really good insulator probably has some good destructive potential, but I'm not sure it would have the necessary kinetic force. Since your main criteria seems to be making a crater, and not just wanton destruction, that means you need a whole lot of kinetic energy.

• Why a container? The god was already 5 Dimensional, just store the air there and "reduce" it to 3dimensions after a certain time Apr 20 at 9:35

Why not just a meteor? As in the Tunguska impact.

It creates an explosion, a sudden release of energy high above the ground, and the vaporized rock disperses.

• Yeah, and you could use exotic elements to boost the power. Osmium is the densest naturally occurring element; it is so dense that a human-sized chunk of Osmium weights 1,400 kg (3,088 lb), which if dropped from 5,000 m yields more than 1 kiloton TNT equivalent energy. Scale up the mass of the Osmium for more yield. Apr 19 at 20:56
• @C8H10N4O2, even Osmium won't do the trick. It's only on the order of 10-20x the density of rock, and that's not enough. See my figures below. Apr 20 at 1:05

A bubble of nothing

Your wizard makes a big bubble inside of which is nothing but a pure vacuum, with water making up the outside of the bubble. You then drop the bubble on the town below. When it pops all that air rushing inwards is going to get very hot, I'm thinking of a scaled up version of Sonoluminescence (which is apparently hot enough to melt steel) so a bubble the size of a building I assume could be hand waved to make quite the bang and thus a large crater.

You might be out of luck here. I don't think there are any explosives that form naturally.

If you really don't want to use gunpowder, you could maybe use ceramics to hold a massive ball of lava, but it would have to come from the immediate surroundings, or it will cool into rock before you reach your target.

I'd personally say the aforementioned sphere of gunpowder is your best bet. Gunpowder isn't hard to make; the most basic form only has three ingredients: charcoal, Saltpeter, and sulphur. Sulphur and Saltpeter occur naturally, and Charcoal is easily made with any trees, some dirt, and a fire.

There's a technique to making charcoal, but it's still pretty simple. You bundle the wood together with as little air space as possible, cover it in a dirt mound, and set it on fire with a few air holes, blocking them off as soon as you can see fire through them. The idea is to make the wood really hot, but cut off the oxygen supply so it doesn't just burn to ash. For much of history, there was a profession dedicated to this: charcoal makers were called colliers.

Your god could simply have given the wizards the recipe.

It's worth noting that Gunpowder technically doesn't detonate; it deflagerates. In order to cause an explosion, it will need to be encased in a solid container to build up pressure. To set it off, use a wax-covered rope; cut it at the right length, set the rope on fire, and drop it such that it explodes without giving the villagers time to put out the fire.

Ideally, you'd make it explode just before it lands, so that the force of impact won't break the container, but if that's too tricky, you could always build a reinforced "landing gear" on the bomb that's designed to break on impact, absorbing the force and leaving the main bomb with enough strength to build up the pressure needed for a massive explosion.

Alternatively, you could drop the bombs with a parachute. You can control how long the bomb takes to explode once lit by cutting the fuse at the right length. you'll just need to keep the prevailing wind conditions in mind.

• Better still, encase the powder in a pointed, stabilized iron shell, so it penetrates the ground some distance before it explodes. Less of the energy is wasted and more goes into cratering. Pretty sure not even a god would get an 800km crater out of gunpowder, though... Apr 19 at 17:47

I would probably drop some acid or a strong base on them. Turn it all into a pile of goo. I'm not a chemist though, so I don't know how likely that is. If that was possible you'd still have to figure out how to "activate it" from 30m away.

I thought about making something shake the ground like an earthquake but I can't see that ever being physically believable. There's just no way to get that much energy out of something that small.

Since you got wizards anyways (sorry, I don't know their powers), you could somehow drop a metal sphere and change its volume or mass and just make it 100m for a while and then go back to regular sized.

Could it be a lifeform? Like they splice together a certain bundle of plants and then it grows out of control for 100m eating everything and then dies because it can't sustain itself. It could have the added benefit of exhaling a bunch of CO2 and suffocating everything around it.

Good luck :)

• "There's just no way to get that much energy out of something that small." There is if you can bring e=mc² into play. You do, at minimum, need either some sort of explosion or a rather magical propulsion system (which doesn't exactly fit with something you "drop"). Or a rather liberal definition of "crater". Apr 19 at 18:18

There is a suggestion that if metallic hydrogen could be created and exposed in a low-pressure environment (i.e. terrestrial atmospheric pressure or lower), that it would not instantly revert to molecular hydrogen, but might remain stable for a short period of time.

If this is correct, dropping a large chunk of metallic hydrogen would likely result in its eventually returning to its molecular state, and then there is the matter of all that hydrogen burning in the oxygen atmosphere.

The density of molecular hydrogen at standard temperature and pressure is 0.08988g/l, or 0.00008988g/cm3. The density of metallic hydrogen is estimated to be approximately 1.15g/cm3, so the change from metallic to gaseous hydrogen would involve an increase in volume of nearly 12800 times... and that doesn't include the expansion as a result of an increase in temperature as a result of the hydrogen combusting.

There may be a few too many 'ifs' in this suggestion, but it has possibilities... especially if you have magic.

Nearest thing coming to mind is a tiny particle of anti-matter.

It is not "stable" by any means and its arrival is bound to create a substantial havoc, but the other requirements should check.

If you cannot remotely create it (make it appear directly at target) then some kind of maglev-in-vacuum container is needed. Any self-respecting God should be able to produce it (actually it's much simpler than obtaining anti-matter).

To trigger explosion it would be enough to let air into container.

• Main issue: not sure how you construct the delivery system without "technology" (as proscribed in the question). Otherwise your crater is going to tend to be at the point of creation, rather than the point of impact. Apr 19 at 16:57
• This is a great answer, but would require a different approach. Instead of dropping the antimatter, the wizards would need to infiltrate the offending city and do the ritual there. They would need fanatical cultists who would be happy to give their lives. It would be some exciting intrigue/spying/witch hunts. Apr 20 at 0:07
• @Matthew Delivery is the easiest - when the container bumps anything, cogs cease turning so the rotating magnetic field stops, perhaps magnets scatter around, perhaps vacuum breaks. Any of these has a consequence that antimatter stops being contained and violently reacts with ordinary matter. It is the container that is difficult to make, but when you have antimatter, also having insulated canister with rotating magnetic field is trivial. Apr 20 at 7:37
• OP said "Natural is key. Avoid Man-made substances/chemicals. The least complex the better.". I'm struggling to see how a magnetic containment field fits that. Apr 20 at 12:32
• "Anti-matter Cultists" are now definitely going to feature in my next D&D sesh :P Apr 20 at 12:36

# Portals

Contained inside the airship are a set of portals roughly 20ft apart, one above the other, magic is used to keep a vacuum between them. Keeping the vacuum and portals going works as a limiting factor. Any sufficiently dense object can then be used to build the necessary kinetic energy. Once sufficient energy is built up, open the bomb bay doors and shut off the bottom portal. Depending on the object used and the speed obtained, the initial slam into the air when the portal magic is ended could destroy the airship.

Since extra dimensionality seems to be a part of your world, connecting two points in 3d space may not be so hard, but keeping the vacuum between them may be. Also, creating 'nothing' might be the riddle that kept the secret locked away for so long.

1/overdose the whirlwind spell: by chance, arrange the runes/preparations too psyonically perfect and the whirlwind doesnt stop until il has dug a hole and thrown the rocks and ground on top of the neighboring city, the whirlwind mutates into an insane toroidal lissajous spiral and bores downwards flinging rocks and buildings far in the air.

2/portal to an asteroid field, like saturn's rings: The whizard does his teleport spell to saturn's rings, and then teleports back in such a way to send a huge rock through a portal at the city

3/earthquake spell

4/subterranean explosion spell, i.e. put 10 tons of magma and 10 tons of water in the same place underground at the same time, the expansion would be so fast that it would cause a mini super volcano caldera.

5/create a void under the city, so that it literally falls down into a sinkhole and everyone slides into the middle of the city and there are just a few arms and legs sticking out of the rubble.