[GUIDE] Detecting tunnels during sieges

[Vysii 77]

Right beneath our feet
An introduction to tunnel warfare
By Ahren Jaeger

This is the second part of my ‘Right beneath our feet’ pamphlet series. In a recent skirmish against the Harrower’s forces, people discovered evidence of the Ailmerians digging a  continent-wide underground tunnel network. The odds are they are not using it for peaceful purposes like setting world records in civil engineering. Allowing them to keep expanding it unchallenged puts the realms of Aevos at grave risk. So I am writing this introductory guide to tunnel warfare to help the people of Aevos more effectively deal with it.

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Table of Contents
Part One: Common uses of tunnels
Part Two: Detecting tunnels
Part Three: Scouting for tunnels
Part Four: Destroying tunnels
Part Five: Tunnel fighting

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Part Two: Detecting tunnels

In sieges, undermining poses a huge threat to even the strongest and most massive of fortifications. It is for very good reason that defenders scramble to intercept enemy tunnels as soon as they figure out that the enemy is trying to collapse the ground from under them. The only way to stop an undermining attempt is to neutralise the tunnel along with its assigned workforce before they reach their intended target. So the moment shovels hit the dirt, the defenders face a race against time to prevent the literal and figurative collapse of their defences.

 

It is thus of utmost importance to detect potential tunnelling activities as early as possible. The more time defenders give themselves to react, the sooner they can commence countermining operations, the less likely an undermining attempt is to succeed. Advance warning is key to survival.

 

This section will explain the most common methods used by defenders of besieged cities and fortresses to detect undermining attempts against their fortifications. These, fortunately, happen to be rather simple and straightforward; they mainly consist of watching for sounds or ground vibrations potentially originating from tunnel construction. Tunnel construction tends to make a lot of sound vibrations, so it is not easy to hide if it is actively being listened for — especially not on the sheer scale the Harrower is attempting.

 

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Sounds and vibrations

To understand how sound-based tunnel detection methods work, it would be useful to have some basic idea of how sound works. Ancient philosophers, architects, and engineers figured out that:

1. sound consists of vibrations passed between contacting matter;
2. a sound moves outwards from its point of origin as a wave (like waves in a water ripple, but in three-dimensional space);
3. a sound loses kinetic energy as it travels through a physical medium;
4. the denser the medium, the faster and farther sound travels;
5. sound loses significantly more kinetic energy as vibrations pass between different types of mediums due to a combination of absorption and reflection.

 

These aspects of sound can be observed, for example, by conversing with another person under different conditions. When someone tries to talk to you, the sound of their voice originates from their lungs and vocal chords before exiting through their mouth. Travelling through the air as its medium, their voice’s sound vibrations travel in all directions as waves. You finally hear their voice when the sound waves enter your ears.

 

If that same person tries talking to you from across a wide field using a normal indoor-voice volume, you will probably not be able to hear them (unless, maybe, you are an elf). But if they try shouting, their voice may just become audible.

This is because sound that is moving has kinetic energy (the energy something has from motion), and it gradually loses kinetic energy as it travels through a medium. But the louder a sound is, the more kinetic energy it has. And the more kinetic energy a sound has, the farther it can travel.

 

So the farther away the other person is standing from you, the more kinetic energy their voice’s sound vibrations will lose by the time they reach your ears; this makes them sound quieter to you until, at some point, you can no longer hear them. But the louder they make their voice, the farther away it can be heard.

 

Another important thing to note about sound is that it does not only travel through air. Sound waves travel through all matter, which could be either solid, liquid, or gaseous in form. The denser the matter, the faster it is for the vibrations to travel through; solids are the densest type of matter, followed in declining order by liquids and gases.

 

But as vibrations travel through changing types of mediums (such from a gas to a solid and vice versa), they lose a significant amount of kinetic energy in the process.

 

If you change the setting of the conversation to opposite sides of an open doorway, their voice will sound loud and clear. But if you shut the door on them, you can still hear them talking even with the door in the way, though their voice will sound quieter and muffled.

 

This is because the sound vibrations of their voice passed through different mediums twice by the time they reached your ears: 1) from air (gas) to wood (solid) and then 2) from wood (solid) to air (gas). Each of these changes in media took away a significant amount of kinetic energy from the other person’s voice. The door, being of a different medium, would have also reflected some incoming sound waves back towards the speaker, refracted the sound waves that made it into the door, and absorbed a significant amount of energy from the sound waves before they once again changed medium to air.

 

And so the other person’s voice is not as clearly audible by the time it reaches your ears. But if you press your ear to the door, or hold a drinking glass to it and press your ear against that, their voice will suddenly become clearer again.

 

For a few more examples of how these aspects of sound can be observed:

 

If you go out for a swim in the ocean and dive underwater, you can hear a whale’s call from hundreds of miles away. But the moment you swim up to the surface and your ears go above-water, you might not be able to hear the whale at all unless it is relatively close. Sound travels farther and faster in water than in air. The boundary between the ocean’s water and the air also reflects the majority of sound waves coming from the whale back underwater, preventing much of the sound from being transmitted above-water.

 

If you are tracking a herd of buffalo across a great plain, you can hear the herd trampling across the ground sooner and from much farther away if you just press your ear to the ground than if you were to try listening from a standing position. Sound travels fastest in solids, so sound vibrations from the buffalo herd will reach the ear touching the ground before reaching the ear facing the air. But the soil, depending on how soft and loose it is, would absorb much of the sound waves’ energy; the sound would probably still be very faint and muffled, but at least you can hear the herd.

 

If you are fighting a magical Voidal entity that drags you into a dimension completely devoid of air, in which there is only an empty vacuum with no matter to transmit sound vibrations, as you start choking to death, no one can hear you scream.

 

Of course, this is a very, very simplified explanation of how sound works. And by no means does it cover everything about it. Sound is a complex thing that scholars of acoustics (the branch of physics that involves the scientific study of sound) devote their entire lives to researching. But even without knowing the specific properties that comprise a sound or any acoustics-related mathematical equations, this basic-level knowledge should be more than enough to help you understand how to detect a tunnel under construction.

 

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How to detect tunnelling

The aspects of sound outlined in the section above enable defenders of a besieged castle to detect undermining attempts, all without necessarily seeing the digging take place. When the attacker’s miners shovel dirt, strike solid rock with pickaxes, or blast stone apart with gunpowder, their actions produce sound vibrations. Sound vibrations from their tunnelling will travel through soil and bedrock, quickly reaching the walls of cellars and other underground rooms within the castle. The vibrations will also reach the castle’s ground-level floors. The defenders can then detect these vibrations through very simple means, triangulate the location of the enemy miners with good accuracy, and dig countermines to intercept them in time to protect their own fortifications.

 

 

Listening galleries

The simplest way to detect tunnelling is to actively listen for it. The best places to intercept tunnelling-related sound waves would be underground rooms such as cellars, dungeons, and cisterns. Once underground, simply press your ear against a wall and it will pick up any sound vibrations travelling through the ground. Better yet, you can hold a glass cup to the wall and listen through the bottom of the glass; the acoustics of the glass would collect, focus, and amplify incoming sound waves coming from the wall, making the sounds more audible. The clearer and more audible the sound, the more likely it is that your ear is facing the exact direction of the incoming tunnel.

 

This is the exact same method people commonly use to eavesdrop on conversations happening in another room. It also works fairly well at detecting tunnelling; during sieges, garrisons would maintain rotating shifts of soldiers or civilian volunteers to man listening posts to give advance warning of enemy undermining attempts.

 

However, this method is not exactly perfect. There is always a fair bit of guesswork involved. And, unless you are listening from a particularly quiet area isolated from unwanted noise, sound vibrations from tunnelling will probably be mixed with vibrations from other sources. In the siege of a city, for example, your ears might pick up ground vibrations caused by other members of the garrison marching on patrol or civilians, horses, and their vehicles moving through a bustling street directly overhead. They would probably sound distinctly different from a shovel hitting dirt or the clink of a metal pickaxe striking hard rock, but all this ambient noise wouldn’t make discerning the sound of tunnelling any easier.

 

This is one of the main reasons why, in more modern fortifications, engineers construct networks of pre-built countermines and underground listening galleries (dedicated rooms for listening in on enemy tunnelling activities) branching out beyond their walls. Far away from urban bustle, these tunnels are less affected by ambient noise and makes it easier to specifically hear the sounds of tunnelling. And so, they generally make for superior listening posts. In times of peace, when there is no imminent threat of being undermined by a besieging enemy, these tunnels also often double as extra storage spaces.

 

Of course, building pre-existing tunnels leading into a fortification carries the risk of besieging enemies either actively looking for these tunnels or mining into them by complete accident. This could, theoretically, provide them a convenient means of entry into the place you’re supposed to be defending. So in anticipation of this scenario, pre-existing countermines are normally deliberately constructed to be confusing and hazardous on purpose to intruders. They are typically built with labyrinthian layouts only the garrison has the time and opportunity to get familiar with, booby traps to stall enemy progress, and explosive charges rigged to block off tunnel sections or collapse them entirely if they are about to be overrun. And so, these tunnels present no great weakness to the defence; the benefits of building them typically far outweigh their risks and costs.

 

[[[OOC: Placeholder image until I figure out how to draw water ripples. Credit: awesomecontent, Freepik]]]

Water ripples

In addition to underground listening posts, defenders of a besieged castle would strategically place down many small water-filled cups, bowls, and other containers at the base of their outer walls and then routinely inspect the water for ripples. Ground vibrations being transmitted through the ground will cause ripples to form in the water. The closer the containers are to a source of strong vibrations (such as those typically created by tunnelling), the stronger the ripples will be. You can estimate the rough direction a tunnel is coming from by observing which containers in which sectors seem to be exhibiting the largest ripples even when nothing is happening nearby on the surface.

 

When inspecting ripples in water containers, it would be advisable to do so when the area is still and quiet. Nearby people walking around, passing horses and wagons, blacksmiths striking their anvils, and heavy construction work, for example, will produce their own ground vibrations, which in turn may produce ‘false positive’ ripples. A few ways to mitigate background vibrations include cordoning off city sectors from foot, horse, and vehicular traffic and enforcing periods of strict silence while ripple inspections are going on.

 

Moats

The classic castle moat is not just difficult or impassable terrain for attackers on the surface. Another, lesser-known purpose of the moat is to hinder enemy undermining efforts. A moat by itself isn’t used to detect tunnelling, but it makes sound and vibration-based methods of tunnelling detection more effective. Moats force enemy miners to dig deeper underground in order to bypass them. To avoid digging into the moat and either exposing themselves to enemy fire or (if it is a wet moat) flooding their own tunnel, miners must often dig into lower, often rockier soil or — in areas with very thin soil layers — even the bedrock layer. Defenders can exploit this to their advantage.

 

By forcing an enemy to tunnel through solid rock, this makes their tunnelling activities far easier to detect and gives defenders more time to plan and react. Tunnelling through solid rock is a lot more energy and effort-intensive than digging through soil, necessitating specialised tools like pickaxes and blasting charges. This, naturally, also produces a lot more noise. Sound also travels more easily through harder and denser solid materials like rock than in softer and looser materials like soil. The end result is that tunnelling sounds become more audible to anyone stationed at an underground listening post.

 

A moat can also prevent undermining entirely. If the moat is dug deep enough to hit the local water table, it becomes virtually impossible to dig a tunnel under the moat without the tunnel immediately flooding with groundwater. The contents of wet moats can also leak into the tunnel. It is theoretically possible to drain away the water with, for example, waterproof wall and ceiling linings and powered water pump and drainage systems. But implementing these would generally be far more cost-intensive and time-consuming than it is worth, especially for tunnels that have a real risk of being countermined and (if they are intended for undermining) will be collapsed anyway,

 

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Recommended actions

To protect important population centres and military installations, it would be within the interests of each realm to ensure its soldiers and town guards are well-informed about the threat of tunnelling and receive formal training in tunnelling detection. If the resources can be spared for it, preemptive countermines and listening galleries should be built underground ahead of any siege, and then — while the Harrower threat is still at large — consistently manned with rotating shifts of guards or civilian volunteers. In areas with thin soil layers, moats should also be dug at least as deep as the bedrock layer. If the water table is fairly high, digging down to the water table would be more ideal. Taking these actions would do a lot to mitigate the threat of the Harrower’s tunnel network. It would be better to do this now rather than waiting until the Harrower marches an army up to your own doorstep.

 

The soil layer across most of Aevos typically seem to be no more than a few metres deep. The Harrower’s legion is likely doing much of its tunnelling deeper underground, somewhere within the bedrock layer. As they are mostly tunnelling through hard rock, their activities likely produce a lot of noise. And given how sound waves travel faster and farther through harder solids, vibrations from their tunnelling should travel far and fast across the land. This should make ground vibrations from their tunnelling activities fairly easy to detect.

 

Because we would be able to detect and react to enemy tunnels fairly early, taking these preemptive measures should greatly diminish the threat of the Harrower’s forces undermining our cities and castles. They will not be able to advance on us from underground without our forces getting a good amount of advance warning and prep time.

 

Unless, of course, the Harrower has mind-enslaved massive, fast-burrowing, and rock-eating Underdark creatures to do most of the digging. Or his forces have dug directly under us already and are just waiting for the best moment to strike from the closest tunnel exit. But in dark times like these we should try to be on the optimistic side.

 

Edited July 4, 2024 by Vysii