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In real life, mines (at their most basic, small containers filled with explosive, fitted with some sort of triggering mechanism, and left to blow up the unwary) are a mainstay of warfare both on land and at sea:

  • Land mines are small, being mainly designed to blow off a soldier’s or animal’s foot or wreck the wheels or tracks of a vehicle (even the large kinds designed to disable heavy armoured vehicles are still fairly small), and usually pressure- or tripwire-activated (i.e., you step on [or drive over] the mine or break its tripwire, mine blows up).
  • Sea mines are much bigger than land mines, being more-or-less exclusively designed for disabling or sinking large ships, and, while primitive types are contact-activated (where you have to actually hit the mine for it to blow up), most use some sort of remote-detection mechanism (where even sailing near the mine is enough to make it blow up).

There is, in real life, essentially no reason to develop naval anti-personnel mines, since humans are creatures of the land, and, even when they fight at sea, they do so with dry feet, from big vehicles that are essentially impervious to anyone in the water - actual fighting in the water itself is quite rare.

Mermaids, on the other hand, are creatures of the water, who live and fight and die in it, and would have very good reason to develop and use underwater anti-personnel mines, if at all possible.

I can already see a number of problems, though:

  • Whereas human soldiers and their vessels are both mostly confined to an essentially two-dimensional battlefield (moving along the surface of the ground and water), mermaids would have the entire three dimensions of the water column to work with (at least until they get far enough down that they have trouble seeing in the dimming light and/or the increasing water pressure starts causing problems); thus, mines planted on the seabed (akin to human land mines) or floating at or just below the surface (like most human sea mines) would be mostly useless. Instead, the mines would have to be distributed throughout the mermaid-useable depth of the water column, which would:
    • Greatly increase the cost and effort of laying a minefield;
    • Most likely require that the mines be tethered to anchors on the seabed (for positive-buoyancy mines) or to floating buoys (for negative-buoyancy mines), in order for the mines to maintain their proper vertical position; as mermaids, unlike human sailors, would be viewing the minefield from within the water itself (without any of the mine-obscuring effects of looking through the air-water interface), this would tend to make the minefield immediately visually obvious.1
  • The aforementioned excessive-visibility problem could be mitigated by hiding the mines in something like a dense kelp forest (assuming that you have a conveniently-located kelp forest or whatnot), but this would bring up yet another problem. One would presumably want to use some sort of contact fuse (probably pressure-sensitive plates or an underwater tripwire), since, unless your mermaids have very advanced electronics at their disposal, most remote-detection detonation systems would be unable to distinguish mermaids from the sea itself (mermaids - assuming that they have a remotely similar biochemical makeup to real-life animals - would have essentially the same magnetic permeability as seawater itself, making them invisible to magnetically-triggered mines, while the sounds of swimming mermaids would likely be very similar to those produced by ocean currents, waves, and other sea life, probably making acoustic mines useless). Unfortunately, contact fuses do not tend to be very discriminating as to what is contacting them (as long as it exerts enough force to trigger the fuse), and would render the mines highly susceptible to being set off by objects moving with the currents. This is not a problem for human land mines (as, even in parts of the world where the ground is prone to moving suddenly, it does not do so particularly often, and the air, while almost constantly in motion, has such a low density that it would take a very powerful storm to move objects heavy enough to set off a land mine), nor for human sea mines (as these are designed to damage or sink large vessels, and, therefore - where they even still use contact fuses at all - require far more applied force than any but the very largest current-borne objects are capable of exerting), but would be quite problematic indeed for mermaids using underwater anti-personnel mines, as a contact fuse sensitive enough to detect a mermaid brushing against it would likely also be sensitive enough to be triggered by current-borne fronds from the kelp forest hiding it.
  • The much lower compressibility of water, as compared to air, would greatly increase the effective range of the shock wave from an underwater mine detonation over that from a detonation of the same size on land; on the plus side, this would greatly expand the mine’s lethal zone, but, on the minus side, it would require that the mines be spaced much further apart than they would be on land, in order to keep a detonating mine from setting off the rest of the field in a progressive sympathetic detonation (and thereby clearing the entire minefield in the process), potentially leaving enough space between the mines for mermaids to swim through without touching the mines at all.
  • Finally, the much higher density and viscosity of water would render fragmentation (a major injury-and-death-causing-and-aggravating factor in land-mine explosions) ineffective as a lethal or injurious mechanism for underwater mines, which would, thus, have to rely entirely on blast injury; the fragments from the mine casing would be slowed down to mere nuisance speeds by hydrodynamic drag within a meter or two of travel, a distance well inside the lethal blast radius for such an explosion.

Given all this, would submerged anti-personnel mines be useful or practical in warfare between mermaids?


1: This would be even worse for mines tethered to floating buoys, as the buoys themselves would likely have to be tethered to the seabed in order to keep them from drifting offsite; thus, you would have an anchor chain or cable extending the full depth of the water column from the anchor all the way up to the floating buoy, plus another chain or cable connecting the buoy to the mine.

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    $\begingroup$ Can you expand on how mermaids in this universe would fight? What kind of weapons do they use? Are the conflicts more raiding / skirmishes on enemy held strong points or is it mermaids meeting in open sea? Also, what level of technology do they have? $\endgroup$ – Jack Apr 25 '20 at 21:10
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    $\begingroup$ The mechanism of land mines is to "blow up the unwary." However the purpose of land mines is to block, disrupt, or channel an enemy force so they will at the wrong place/time for their plan (but the right place/time for the opposing force). The mechanism of a weapon is generally less important than the purpose and proper employment of the weapon. When the war is over, the winner will need to justify pointless slaughter and expense to themselves if they want to keep believing they were right. Use of mines as a non-military (terror) weapon is reprehensible...and criminal. $\endgroup$ – user535733 Apr 25 '20 at 21:47
  • $\begingroup$ Obligatory link: youtu.be/bgUu8uLhdDk?t=30 $\endgroup$ – Damon Apr 26 '20 at 19:00
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    $\begingroup$ >mermaids - assuming that they have a remotely similar biochemical makeup to real-life animals - would have essentially the same magnetic permeability as seawater itself, making them invisible to magnetically-triggered mines I don't think that's true, since sharks can sense magnetic fields, and use this sense to hunt real life animals: en.wikipedia.org/wiki/Shark#Electroreception. $\endgroup$ – alexgbelov Apr 27 '20 at 13:33
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    $\begingroup$ Sure. Make the mines look like 4 inch in diameter clam shells. Then when the mermaids try to pick up the shells to make bras KABOOM. $\endgroup$ – MaxW Apr 27 '20 at 20:30
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Mines would be much more useful against mermaids.

  1. As OP notes, the incompressibility of water makes explosives more deadly underwater. The shock wave kills at some distance; this is the principle behind fishing with dynamite.

  2. As area denial weapons, mines are better in the water than on land. On land, these weapons persist in the area for years after they are needed or wanted, causing persisting harm. In the water, untethered mines will almost all have been triggered by chance events within a few months of having been deployed. Unless you are Finding Nemo and you stumble into a trove of dozens of unexploded WW2 mines.

  3. Mermaids are intelligent. They will investigate interesting things in the water which includes interesting mines or disguised mines. Some people assert the Soviet mines dropped over Afghanistan were made of brightly colored plastic to attract investigation, which then detonates the mine.

https://www.scientificamerican.com/article/drones-used-to-find-toylike-butterfly-land-mines/

One would think people would figure out those things are bad and not to touch them. The ones that do don't touch them. It helps that those Soviet mines all look the same. If you randomized the appearance it would be harder to figure out - the IED roadside bomb principle.

  1. Different neutral buoyancies. The Russian mines were dropped from helicopters to litter large areas. Your anti mermaid mines could work similarly, and float in the water column at varying heights according to density. This would provide area denial over 3 dimensions.
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    $\begingroup$ Brutal and really efficient. love it. $\endgroup$ – Gustavo Apr 26 '20 at 1:05
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    $\begingroup$ Please tell me those attractive mines were looked on dimly by the Geneva convention! $\endgroup$ – John Dvorak Apr 26 '20 at 9:34
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    $\begingroup$ @JohnDvorak The geneva convention prohibits "the targeting of civilian population" since '77. The additional protocols ('80/'83) restrict their use, and in '97 the mine ban convention outlawed them altogether. HOWEVER, all but the first of those only affect signatories - which the US, Russia and China are notably not. The US declared in 2014 they would abide by it except in Korea, but in 2020 under Trump they removed restrictions on using anti-personell land mines mostly. So, most of the world agrees that landmines suck in general, but the big players don't want to lose their evil toys. $\endgroup$ – Syndic Apr 27 '20 at 7:52
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    $\begingroup$ @Syndic Evil is as evil does... $\endgroup$ – I'm with Monica Apr 27 '20 at 9:05
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    $\begingroup$ @JohnDvorak, as a person born in USSR and grown up during Afganistan war, I can tell you that for soviets this was simmetrically opposite. We were told that mojaheds were masking mines in common objects (lake cola cans or audioplayers) and leaving them for soviet soldiers to trigger. I remember how we were afraid of picking up such things even on our own streets! So I don't know what truth is. But in our "military lessons" at school we were tought norms of Geneva convention and what military crime is. $\endgroup$ – ksbes Apr 27 '20 at 12:13
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Anti-swimmer grenades already exist; as noted, shockwaves are much more effective underwater, shrapnel much less effective.

An antipersonnel underwater mine would likely consist of a similar warhead with suitable fusing and/or guidance (almost certainly sonar, either active or passive).

Note that visibility is generally very low underwater and mines are unlikely to be detected visually.

Mines can easily be disguised as floating debris. Tens of thousands of tons of floating fishing gear are discarded, lost or abandoned at sea every year, and mines could easily be hiddien among them. Alternately, they could use biomimicry.

Which leads me to ask - have you seen the Wattozz underwater swarming manta ray mine?https://www.popularmechanics.com/military/research/a20136004/robot-stingray-turkey-wattozz/

enter image description here

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    $\begingroup$ Anti-swimmer grenades already exist; A link, please? Even if they are, I reckon they'll be dropped from the surface or be a variant of mini-torpedoes - simply tossing a grenade under water toward the enemy (like on land) is suicidal. $\endgroup$ – Adrian Colomitchi Apr 26 '20 at 9:14
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    $\begingroup$ Anti-swimmer grenade -- defense-aerospace.com/article-view/release/86498/… -- my point was that the warhead technology was well understood, as the the technology of underwater mines, it's just a matter of appropriate fuzing/guidance for a mine $\endgroup$ – David Hambling Apr 26 '20 at 10:40
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    $\begingroup$ Re: "a matter of appropriate fuzing/*guidance* for a mine` I reckon that's a matter of terminology, as I don't expect a mine to be mobile on its own. If we include the mobility, I'm afraid that we are stepping outside the boundary of the question ("containers filled with explosive, ..., and left to blow up the unwary"). Otherwise, good info, thank you for it ( + suggesting would worth putting the link into your answer) $\endgroup$ – Adrian Colomitchi Apr 26 '20 at 12:43
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    $\begingroup$ Some underwater mines are mobile. Some, like the US Navy's Submarine Launched Mobile Mine move then than lie in wait; others like the US navy's CAPTOR and new Hammerhead lie in wait and then fire a homing torpedo rather than just exploding. Some, like the Turkish Wattozz in the answer are mobile before and after detecting the target. $\endgroup$ – David Hambling Apr 26 '20 at 15:54
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You want mines that trigger at a particular target. You can use a variety of detonating methods to kill your intruders with varying amounts of sensitivity to other types of debris or roaming fish. So what you do is create large nets.

1: For a minefield with the least chance of murdering random debris or large fish you create a bunch of nets that make traversal of the area hard for most fish. The nets can form a (3D!) maze to pass through, potentially causing anyone inside to starve or be discovered during the time consuming task. Or you can cut through the nets... except that some of the nets contain explosives that detonate if you cut through their netting. The nets can ofcourse be created of thin, tough wiring making it far harder to see the nets in advance, or which net that explosive is attached to.

2: small surface-area nets are spread around the area using a variety of anchors or neutral bouyancy bouys at different heights. The nets are stiff, but a sufficiently large deformation will cause the bomb(s) attached to the net to go off. A small fish or debris will not have the force to do this, but a large mermaid at basic speeds that would likely hit it with his full body would deform it enough to detonate the mines.

The holes of the nets would be fairly large so most smaller fish and debris can simply pass through (think chickenwire or larger holes). A non-explosive and very visible net would be used as warning against non-mermaids/civilians to keep the minefield intact.The wiring can be different per net. A thick wiring could be used to steer enemies away into thinner and less visible wiring.

Nets would be ideal for area-denial, especially since mines cant be placed too close to eachother. The point of mines isn't necessarily to kill but to deny access. If you really want to kill something you release floating mine-nets to drift towards your opponents.

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    $\begingroup$ Not sure how effective this would be, simply because there are lots of mermaid-sized (or larger) oceangoing critters out there that would trip these. Whales, sharks, dolphins, rays, and sunfish, to name a few. $\endgroup$ – Doktor J Apr 27 '20 at 14:27
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    $\begingroup$ @DoktorJ the same could be said for anti-personell mines on land. Many animals can trip these mines as easily as children, civilians and soldiers. $\endgroup$ – Demigan Apr 27 '20 at 15:00
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It's worth noting that the purpose of mines isn't necessarily to kill, but rather risk of death or injury is the method they use to fulfill their primary purpose, which is area denial.

On land, mines need to be invisible because if they are visible they are easy to avoid, but mines can fulfill their purpose when visible as well as long as they are not easily avoided or removed. By having them close together you force the enemy to move through the minefield slowly and still with a high risk of accidents, which is good enough to make them useful.

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Why not use the example of nature, and use the example of sharks, which have an electrical sense that can detect the electrical signals emitted by contracting muscles. By emplacing mines that are capable of detecting the muscle activity of a mer-person-sized being, they need never be touched in order to detonate.

Unlike an antipersonnel landmine, which is really intended to cause non-fatal damage to the lower limbs (thus consuming more of the enemy's resources than a corpse) by projecting shrapnel into them, an underwater antipersonnel mine would function by emitting a shockwave that would damage the internal organs of the merfolk nearby. In humans exposed to atmospheric shockwaves, the organs most likely to be damaged are the lungs, followed by the brain and gastrointesinal tract. See https://en.wikipedia.org/wiki/Blast_injury. However, shrapnel is also involved, whether generated as an intended component of the explosive device or incidentally by propulsion of nearby debris.

Underwater, shrapnel effects are minimal, but the effects of shockwaves are much greater. This means that an underwater mine has a much greater area of effect for a given amount of explosive. An analysis of underwater blasts on humans has shown that the potential for lung and gastrointestinal injuries is very much greater, as is the potential for auditory damage. See https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4659630/.

Now, I must speculate on the physiology of mer-people. It may be assumed that mer-people would have a gastrointestinal tract, though whether they have gills or lungs is a matter of interpretation. However, the effects of an explosion upon gills would likely be similar to the effect upon lungs.

This means that as an area-denial weapon, underwater mines would require a lower density of explosives than the volume of water might suggest. There could be a number of different ways of emplacing mines, including tethered floating mines and sub-surface mine launchers.

Tethered mines would likely work best in shallow water, where a mine suspended half-way between the surface and the sea bed would have a sufficient blast radius to reach the surface and sea floor. Combined with an electrical impulse detection detonator, this would very effectively deny the area to the enemy. In order to prevent false positives or anti-mine devices from detonating a mine out of its effective area, mines could be networked so that they could detect a source of electrical activity and triangulate its location, and only detonate the mine closest to the electrical source when the electrical source was in effective range. This would make mine-clearance particularly challenging.

Sub-surface mine launchers could be buried in the sea-floor in deep water. When electrical activity is detected in the volume above the minefield, its position can be triangulated, and a launcher beneath the signal activated, setting a buoyant mine to detonate at the depth of the signal before releasing it. This system would also be difficult for wartime mine clearance to deal with. The launchers could easily contain multiple mines, so that the minefield wouldn't be exhausted by the launch of a few mines.

This system could be triggered by other large marine life-forms such as large fish or marine mammals. However, it could be marked by a tethered floating platform that uses wave action to sound an underwater bell sufficiently loud and distinctive to deter sea life from approaching, also providing an audible warning to mer-people that mines were in the area... even if in fact no mines actually existed.

The complication with mine clearance would be that an artificial electrical field would have to be close enough to detonate a mine, a distance that might be well-within the mine's range to cause injury to a mine-clearance crew with a neutrally-buoyant electrical device on a long cable.

It might also be possible to coast through a minefield on a sea-current while making no exertion that would cause a sufficiently large electrical signal to trigger a detonation, but by introducing obstacles to a minefield, such that muscular activity would be required to avoid them, this can be mitigated.

So, it can be seen that an underwater minefield can be just as effective, if not more so, than a minefield or even a simple "Danger! Mines!" sign on land, and potentially not greatly different in the cost of emplacement.

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    $\begingroup$ Considering that "deep" water might be multiple kilometers deep, I'm not so sure about the effectiveness of an anti-personnel torpedo system that launches from the sea floor. $\endgroup$ – nick012000 Apr 27 '20 at 7:01
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    $\begingroup$ Depending on the technology level, it may be possible to create a propelled device that generates the appropriate electrical impulses and send it into the minefield. You need not use motors for propulsion (compressed gas, for example), and as long as its body is built aquadynamically enough to ensure it continues on a relatively straight trajectory, you can launch it from several hundred yards/meters away. $\endgroup$ – Doktor J Apr 27 '20 at 14:29
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No. Underwater antipersonnel mines would be fairly ineffective for the same reason aerial mines would be ineffective - Water and Air are 3 dimensional spaces whereas land is essentially a 2 dimensional surface.

To achieve the purpose, the underwater AP mines would have to be somehow capable of detecting over a spherical surface, and they'd have to be positioned to cover a wall from sea-floor to surface, or to cover an area, with sufficient overlap to prevent anyone swimming between them.

Also the AP mines would have to be positioned "thick" enough that one detonation doesn't leave a hole, but far enough apart that one detonation doesn't explode adjacent mines.

I'm excluding powered or self-propelled mines, because that would not be a mine, it would be an autonomous hunter-seeker missile or drone.... which could be quite effective if they were smart enough.

There are no air-mines hanging in the sky to explode the enemy's planes - instead we have missiles. The nearest thing would have been barrage balloons filled with explosives in WW1, and they were a strategic target to attack in their own right, just booby-trapped.

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Minefields aren't used in most wars; quite apart from them being ethically difficult, they can be a problem for your own side later, and require resources to deploy. Whilst devastating for civilians, they're also relatively easy for the military to ignore – vehicles can be immune to AP mines – or to move around.

However, mines and IEDs are still in common use. These are placed carefully, in small numbers. They'd be as effective in urban areas for merfolk as for humans (and moreso, due to the shockwave underwater). Methods of triggering would be largely similar – manual and tripwires being popular, and pressure for anything that's going to be touched (doors, boxes, etc).

Pressure sensors (of the step-on / drive-on type) won't work, but in inside spaces where noise and water movement would be lower, detecting a swimmer should be easy – fish can do so. Detecting vehicles by cavitation is easy.

Also, spotting mines would probably be difficult for merfolk. Visibility underwater is often poor, and any silt will make it very hard to see. Except for the top few metres, we should not expect merfolk to be able to see underwater anywhere near as well (accurately) as we see on land – even with specific adaptions. So things like fishing line would be very close to invisible, making tripwires/nets very effective.

Shockwave needn't cause premature detonation, depending on the types of explosive used. The pressure level required to maim or kill a human/merfolk would be much lower than required to detonate most explosive.

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  • $\begingroup$ Vehicles being immune to antipersonnel mines doesn't solve your problem unless you also have infantry that're immune to antipersonnel mines. $\endgroup$ – Vikki Apr 27 '20 at 20:45
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The answer to your problem is CAPTOR mines... Technically they are miniature torpedoes (CAPTOR = enCAPsulated TORpedo) with a sensor that triggers the torpedo to go after a target that came into range. Drop them in place, and they'll watch an area.

And yes, modern navy consider them mines.

https://en.wikipedia.org/wiki/Mark_60_CAPTOR

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