[GUIDE] An introduction to tunnel warfare

[Vysii 77]

{!} In a smoky corner of the Basking Turtle tavern in Beznov, Adria, Ahren Jaeger lay slumped over his usual table, seemingly half-dead. An inordinate number of empty Euler Max bottles lay strewn about the tabletop. Traces of smoke rose from the tip of his quill. This meant only one thing: the siege engineer was writing another essay.

Some time later (after taking some herbal remedies and alchemical concoctions for his liver) he would travel around the various realms of Aevos, distributing printed copies of the following essay series to any public library that would accept them. For more centralised discussion, he would also pin a copy in a corner of the Adrian Quest Board along with a blank sheet of paper for people to write feedback on. {!}

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Right beneath our feet

By Ahren Jaeger

The Harrower readies his next strike from right under our very feet. Deep below the soil on which we stand, his forces are digging the world’s largest network of underground tunnels. He is likely using artificial tunnels, along with natural sections of the Aevosian Underdark, to move troops, equipment, and supplies around the continent undetected and relatively unopposed. He and his forces can wreak a lot of havoc with such tunnels at their disposal. The only thing limiting their sheer destructive potential is human imagination.

 

The Harrower’s tunnel network could pose a serious threat to the realms of Aevos if we fail to enact timely and effective plans to neutralise it. I thus urge realm leaders to prepare their troops for tunnel warfare and commence countermining operations as soon as possible. It appears that the Harrower’s forces have already made a lot of construction progress, so it might not be very long until they do something truly terrible. We must do something to prevent (or at least soften) the coming blow.

 

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Preface

As of the time of this pamphlet series’ publishing, there exists a powerful Darkspawn entity known as the Harrower. He appeared a few decades ago, enthralling the tribes of Ailmere under a wicked curse and enshrouding the northern taiga in a fog of voidal darkness. He still leads his legion of brainwashed Ailmerians in a war of subjugation against the rest of Descendantkind.

 

The Harrower’s forces suffered no few costly defeats over the course of the war, and have since retreated back to the northern taiga to lick their wounds. But while, on the surface, it might look like they’ve put their campaign on pause, it would be a mistake to assume they’ve been lying dormant. After a skirmish involving the Harrower’s elite Chosen warriors, a concerning map was found on the body of one.

 

This map, which consists of intricate carvings on a stone tablet, suggests the existence of a continent-wide underground tunnel network. It also implies a new, potentially more troublesome phase in the Harrower War. The Harrower War will probably feature quite a lot of tunnel warfare going forward. It isn’t something that happens very often in Aevos. And it’s reasonable to assume most fighters in most realms aren’t very experienced in it. Tunnel warfare requires a very different approach and mindset to more conventional warfare on the surface. So I am writing this pamphlet series to help people understand it better.

 

The main goal of this pamphlet series is to equip the leaders, soldiers, adventurers and prospective combat engineers of Aevos with some basic knowledge of how tunnels are typically used in warfare and some idea of how to go about detecting and countering them. So when the Harrower tries invading our homes from below, at least we won’t be completely blindsided.

 

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An introduction to tunnel warfare
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 One: Common uses of tunnels

The pickaxe and shovel can be more devastating tools of war than any blade or cannon. Descendants have been weaponising tunnels in warfare for thousands of years, and often to decisive effect. They’ve brought down grand fortresses where massed artillery bombardments failed. They’ve empowered rag-tag bands of insurgents to fight much greater armies to a standstill. And more. The mundane hole in the ground can determine, and has determined, the course of entire battles. The utility of tunnels should not be underestimated.

 

This section will go into some detail about a few of the more common military applications of tunnels. In siege warfare, engineers dig tunnels to undermine well-defended and artillery-resistant walls. In guerilla or asymmetric warfare, tunnels can benefit numerically and technically inferior forces as excellent force multipliers. They may dig tunnels to stealthily manoeuvre around an area, allowing them to flank and ambush superior enemy forces. They can also act as hidden bases of operations and as a strong means of area defence. Knowing that these applications exist and why they work will be a good starting point for understanding the potential threat posed by the Harrower’s tunnel network and how to go about dealing with it.

 

Undermining

Some walls are so heavily reinforced that they will never topple over no matter how many rocks or cannonballs you lob at them. In these cases, siege engineers can still breach those walls by using an age-old tactic called ‘undermining’.

Undermining basically consists of digging a tunnel directly underneath a building’s foundations, building temporary supports to prop up the structure, and then destroying those supports (usually either by burning them away or blowing them up). A combination of gravity and mechanical stress will then cause the building above to fall into the tunnel.

 

The tactic features heavily in sieges, in which walls are its usual target. When the Attacking side undermines a wall section, the resulting collapse often creates a huge breach in the defences through which their troops can assault the interior and eventually overwhelm the Defending side with superior numbers. The Defending side, having just lost their main advantage of fortifications, often surrenders before they get slaughtered.

 

The threat of undermining should be a compelling reason why fortress commanders and city rulers should keep around dedicated teams of engineers and miners in times of war. Oftentimes the only viable ways for them to stop a besieging force from tunnelling under their walls are to either 1) dig a moat deep enough to reach the local water table or 2) build their own underground tunnel or ‘countermine’ to intercept and destroy the enemy’s before they can get too close.

How to undermine a wall

The basic process of undermining a wall hasn’t changed too radically over the millennia. The besieging side’s engineers figure out exactly how far away the target enemy wall section is, calculate how big a tunnel they need to dig and how much kindling or explosive ordnance they need to achieve a breach of a desired size, and then plot a path for their team to dig. Miners then dig into the ground with shovels and pickaxes, deep enough to avoid exposing themselves to enemy fire on the surface, but not so deep as to hit the local water table and end up flooding their entire tunnel with groundwater. Along the way, carpenters make shorings and supports to hold up the walls and ceilings. The team then sets up these supports, hauls debris to designated dumping areas outside, and keeps careful track of their precise location in relation to the target wall section. When they end up directly underneath the target wall section, they then destroy the supports under it while maintaining a safe distance. The resulting cave-in finally collapses the wall above.

 

The two most popular methods for initiating the collapse involve the use of fire and explosives. The first method involves simply starting a large fire underneath a target structure to burn away the wooden supports. The second method replaces the slow-burning fire with a gunpowder explosion contained behind a sandbag wall. A small explosion should be sufficient to near-instantly destroy the tunnel’s supports. A larger explosion involving massive amounts of gunpowder can more thoroughly weaken or destroy a structure’s foundations, shake the entire structure at its very core, and potentially achieve even bigger collapses.

The effectiveness of undermining compared to artillery bombardment

It is perfectly possible to build a wall so thick and shock-absorbent that even cannonballs will bounce off of it, dealing no serious damage. Seeing this, it’s no wonder why the rulers and inhabitants of cities with such walls so often boast of their strength. They may even go as far as to claim their walls are invincible. This is a common folly. For while they can resist artillery bombardment, no land-based structure is completely invulnerable to undermining.

 

At the moment, cannons are the most powerful mundane artillery available to Descendantkind. They use the explosively expanding gases released by burning gunpowder, funnelled through the narrow and partially enclosed confines of a gun barrel, to launch projectiles at high speeds and with great force. The destructive power of the average siege bombard far surpasses that of the average trebuchet; a wall that takes older siege engines months to destroy can be levelled in a few days by cannons. And so, cannons have largely replaced classical siege weapons when it comes to tearing down fortifications. But even gunpowder, mighty as it is, has its limits.

 

By far, the most common type of cannon ammunition is a solid and somewhat loose-fitting spherical projectile called ‘round shot’ or ‘solid shot’. The destructive power of a cannonball (assuming it’s already strong and hard enough to not break apart or deform heavily on impact) mainly depends on its kinetic energy (the energy something has because of its motion), which is proportional to its mass and the velocity at which it impacts its target. Basically, the heavier the cannonball and the faster it is flying, the more damage it deals when it finally hits something. Cast iron cannonballs and corned powder have rendered many classical fortress designs practically obsolete, but the combination remains far from a be-all-end-all.

 

Gunpowder can only propel an object so fast. And there also seems to be a soft limit to the weight of ammunition gunpowder is capable of launching at practical ranges without also blowing up the cannon and its crew. The very largest of siege bombards and mortars on Eos can fire stone or metal projectiles weighing a little more than a metric tonne. While these can utterly obliterate most classical-style curtain walls and castle towers with ease, defence-minded military engineers developed many innovations to mitigate the effectiveness of artillery. For example: solid shot fired from these same cannons would struggle against the thick and shock-absorbent brick-and-earth walls of a modern bastion fort or Oyashiman ‘shiro’ (castle).

 

Of course, it is possible to make specially crafted ammunition or ‘Craftsman’s Shot’ that have more going on for them than just their kinetic energy. For example, engineers may incorporate explosive charges using low-density boomsteel or blasting potion gas. But these tend to be prohibitively difficult and expensive to manufacture at scale. So militaries generally stick with solid shot for the most part.

 

When even concentrated artillery fire proves lacking, siege engineers must find other ways of achieving a breach. And oftentimes they resort to undermining. Against artillery-resistant walls, simply collapsing tunnels directly beneath them are often what ends up creating the breaches a besieging army needs to finally take a fortress by storm. This is because undermining weaponises the unrelenting force of gravity and turns a structure’s own (often huge) mass against it. At least for now, there are very few man-made weapons that can compare.

Destroying large structures

Various forces constantly act upon the things we build, even when we’re not actively trying to destroy them. To specify a few: Gravity always exerts a downwards force on our buildings and seeks to sink them into the ground. Wind blows against their walls, wobbling taller structures and constantly threatening to knock them over. The materials we make them out of also need to be able to withstand the ever-shifting weight of their moving occupants and the things said occupants put inside them. In areas prone to natural disasters like earthquakes, flooding, and hurricanes, they also need to be specially reinforced with Acts of God in mind. And of course, any building needs to be able to support its own weight so as to not fall apart on its own.

 

One of, if not the most, important contributors to what keeps a building standing is its foundations. Foundations are critical to structural integrity: they bear the structure's load, they safely and more evenly transfer its weight onto the ground, and they stabilise it against horizontal movement. Without good foundations and a stable connection to the ground, larger and heavier structures will eventually sink into the soil and crack themselves open without any outside intervention. The job of the foundations becomes more difficult as the mass they must support increases.

 

As a structure is built bigger (and thus heavier), its foundations must be exponentially stronger to support its weight and prevent the entire structure from collapsing. This is because of the Square–Cube Law: if you double the size of, say, a cube, its surface area is quadrupled, and its original volume is multiplied by eight. Essentially, as a structure grows linearly in size, its mass grows exponentially. So the larger a structure is built, the more it will take to keep it standing in the face of the various forces and stresses constantly seeking to displace it. And at some point, enlarging a structure becomes counterproductive; it becomes more prone to collapse as the inherent strength of its constituent building materials (or at least the ones available with our current level of technology) gets pushed to their limits.

 

Undermining basically deletes foundations from the equation. Remove the ground below a target building section, and the only things left to hold it up are its connections to adjacent building sections that are still touching the ground. The entire weight of the undermined section continues acting as a strong downward force. But without any connection to the ground to support it, and the weight of the foundations themselves now heavily contributing, it puts itself and adjacent building sections under extremely high amounts of stress. The bigger and heavier a building, the greater the downwards force it will exert on itself, the greater the stresses it imposes on its own structure and constituent building materials. With larger and heavier structures, like castle or city walls, the inherent material strength of even strong building materials like stone brick often proves to be no substitute for solid foundations. And so, the undermined building section collapses, often taking parts of adjacent sections along with it. Sometimes, this may even start a domino effect that inflicts even more devastation against other parts of the building.

 

Even the grandest of castle keeps, the tallest of city walls, and most imposing of evil villain towers are still very much susceptible (or are perhaps even more vulnerable) to undermining. No matter how strong people believe their walls to be, no one should ever feel complacent. The ground may just open up right under them.

 

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Guerilla warfare

Tunnels feature prominently in guerilla warfare because it helps insurgents operate out of sight and often out of mind of enemy forces, which tend to be numerically and technically superior. As soon as guerillas get caught in a prolonged engagement or pitched battle, they typically get annihilated. They only manage to survive in the long term by avoiding battle until they find good opportunities to inflict great damage on the enemy while minimising their own losses. To that end, forces finding themselves fighting at a disadvantage often find great utility in tunnels.

 

Tunnels as covert avenues of movement

Underground tunnels allow guerilla forces to covertly move around fighters, equipment, and supplies without being noticed by enemies on the surface. If guerillas move around on the surface, they risk getting spotted and engaged even if they’re covered head-to-toe in camouflage. However, if the very ground is blocking direct line of sight and muffling sound, an enemy patrol may pass by on the surface completely oblivious to the fact that there’s an entire tunnel system in active use mere metres beneath their feet. When travelling on the surface is too dangerous and risky, tunnels can be a very stealthy and safe way to get from Point A to Point B.

 

This can be exploited both offensively and defensively. Tunnels can enable fighters to covertly outflank their enemies and strike from various and unexpected angles and locations. In raiding operations, a tunnel could let them bypass enemy watchtowers and fortifications, strike targets deep inside enemy territory, and vanish before any large muster can be rallied against them. In the defence of an area, tunnels can be especially useful tools.

 

Tunnels as underground strongholds

With tunnels, a military force can maintain a powerful and prolonged presence even deep inside enemy territory while staying hidden. Engineers can expand rudimentary underground tunnels into veritable underground fortresses. Such tunnels may have all sorts of amenities such as sleeping quarters, storerooms, hospitals, training areas, latrines, recreational rooms, deep wells, and kitchens. A tunnel can be expanded so that even large military units can comfortably live within and operate from them for extended periods of time. If a tunnel network is allowed to develop to such great size and complexity, it can become extremely difficult for attackers to dislodge enemies from them.

 

Underground tunnels also give great advantages to its defenders, making them difficult for attackers to clear out. Situated beneath layers of soil and perhaps even solid rock, they are naturally resistant to artillery bombardment and aerial attack. Through the use of trap doors, air locks, water locks, and ventilation pipes, larger and more complex tunnel networks can no longer be destroyed by simply bellowing fire or poison gases into an entrance. The narrow confines of small tunnels also make it easy to funnel intruders into booby traps like punji sticks, venomous animals and insects, and pitfalls.

 

Adding to the difficulty of clearing out such tunnels, fighters who inhabit a tunnel system will likely grow intimately familiar with all its nooks and crannies over time. In defending it, they will use their superior terrain knowledge to their full advantage. It is also common for long-time inhabitants of a tunnel network to run drills simulating defending it in complete darkness. They may train their senses of smell and sound to extremely keen levels, such as to the point they can differentiate friend from foe just by how one moves and breathes around the environment. Attackers will often find themselves taking heavy casualties to traps and ambushes.

 

Besieging a tunnel network and trying to starve out the defenders can also be rendered an ineffective tactic. A tunnel’s occupants may have tunnel sections unknown to the attackers or build entirely new branches leading outside the cordon. These could be used by the defenders for resupply or escape. They could also be used to launch sorties from beyond the cordon, allowing them to strike the attackers from the more exposed rear. When operating in an area where the enemy is known to use tunnels, one should never drop their guard.

 

If such well-developed tunnels are encountered, there is typically no ‘easy’ way to get rid of them. The odds are that troops will need to delve inside and eliminate the tunnel’s inhabitants in hand-to-hand combat. And unfortunately for the attacking side, tunnel fighting tends to be a very brutal and messy affair compared to combat on the surface.

 

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Predictions for the Harrower’s strategy

With uncontested control of the underground realm, a continent-wide underground tunnel network opens up a whole new world of tactical and strategic possibilities for the Harrower’s legion. Their new offensive strategy will likely feature the tunnel uses described above.

 

The Harrower’s legion will likely build a number of underground bases — at least one stronghold in each realm they're targeting. Given their current less-than-successful track record of invading Aevosian lands, any forward operating base they build on the surface outside of Ailmere will probably get razed within months of them breaking ground. But if they build their bases underground, they will be a lot more difficult for Aevosian forces to detect and destroy in a timely manner. They can developed into excellent staging areas for offensive campaigns in relative safety. They can also make hidden supply depots to sustain large forces far away from Ailmere. And even if these bases do get discovered, clearing them out will be easier said than done. This is probably their best bet at projecting power beyond the borders of their enshadowed homeland, at least until they can regain enough strength to capture and hold significant territory on the surface.

 

Tunnels would also enable them to wage asymmetric warfare campaigns within the realms of Aevos. For example: they can attack merchants and travellers on the roads, raid lightly-defended settlements, and ambush troop columns on the march before disappearing back underground. They would also be more successful than average at kidnapping people to create more mind-slaves for the Harrower; nearby tunnels entrances would be a very convenient means of escape. With escape routes more conveniently close-by, they can muster the confidence to pull off all manner of other shenanigans. With tunnels, the Harrower’s forces can cause a massive amount of damage and chaos by the time quick response forces arrive to stop them.

 

Undermining will also probably play a key role in sieges. The Harrower’s forces can branch out from their main tunnels and dig up towards the base of any fortification that gives them trouble (should the local terrain permit it). They will probably rely on undermining a fair bit because don't have many other viable options for breaching the average city wall. It seems as if they have very limited access to gunpowder; instead of cannons, they rely on older and weaker siege weapons like trebuchets and onagers. Theoretically, they could match the firepower of siege bombards and mortars by building absurdly large trebuchets capable of launching massive boulders. But such projects would, realistically, be destroyed by fire arrows, sallying forces, or cannonballs long before they can be completed. Alongside the First Zenith’s Portal magics and the Chosens’ Shadow magic teleportation, tunnels are one of their few practical weapons for sieging down our cities.

 

Things get a lot more complicated and unpredictable when you throw magic into the mix. The Harrower is an extradimensional Voidal entity whose powers are continuously growing and expanding. And among his followers – or at least the ones that we currently know of – are a number of powerful sorcerers: he commands Frost Witches, Necromancers, his Shadow Magic-wielding Chosen, and several generals (called Zeniths) with unique magical abilities such as Portal summoning and Self-cloning. Together, they can probably brainstorm and pull off any number of creative, unexpected, and devastating stunts by pooling their spells and combining them with the utility of tunnels. But rather than theorising what they could do, it would probably be more productive to try and physically shut down their ideas before they have the chance to surprise you.

 

The Harrower’s tunnel network can not be ignored any longer. His tunnel warfare strategy poses a massive security risk to every realm of Aevos. If the stone tablet map is anything to go by, his legion already made quite a bit of progress. We must contest their control of the underground realm before it can be solidified any further. The more we idle, the more we undermine our own safety.

Edited June 26, 2024 by Vysii