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Setting

A humanoid race with advanced technology, on an earthlike planet, has an intruding alien presence underwater off the coast of one of its major port cities. The aliens are animalistic, ravenous, deadly, hive-based, fast-breeding and amphibious. The humanoids have organized armed forces, tanks, spaceships, etc.

Event

The amphibious aliens are pouring out of the ocean to attack the port city, in a continuous wall of oversized teeth and razorlike claws. The humanoids are entrenched on the city limits, with temporary fortifications, mobile laser batteries, and ships hovering above to rain fire down. They can keep the aliens at bay with effort, but the nearby ocean is completely teeming with hostiles, and any attempt to enter the water and eliminate the hives on the ocean floor has failed. For now, it's a stalemate.

The Problem

In such a balanced scenario, the main concern of a humanoid commander would be flanking attacks. Since the aliens are completely comfortable in the water, there's nothing preventing them from spreading out along the coastline and attacking the city from other angles--or even crossing the ocean and assaulting more cities on other continents. If the humanoids can't eliminate the aliens fast, they will need to contain them or the entire planet will eventually be overrun. How does one do this?

Suggestions

I have received a few suggestions, most of which hinge on some sort of weakness that would be discovered in the aliens:

  • Sonic-based containment buoys
  • Chemicals in the water
  • Somehow removing the air from a section of the water

However, these constitute attack vectors which could be used to simply and effectively eliminate the entire threat. What is needed (for storyline purposes) is something that can contain but not kill the aliens.

(A decent suggestion has been planting some sort of lure in the port city so that the aliens are not tempted to spread out through the ocean, but I believe they're a little too smart for that, and besides, what lure? I don't believe they have some sort of catnip equivalent.)

Best Idea

My best idea so far has been going out beyond the perimeter of the alien presence and using some sort of technology (bombs, drills) to create a large fissure in the crust of the planet, with the idea that you're basically making some kind of volcanic-based wall: lava, superheated steam, etc. in a large line encompassing the aliens.

Obviously this would have potentially serious ramifications on the planet, generating earthquakes and so forth, but if the humanoids are desperate and have no other options, they might be willing to go to these lengths.

My uncertainty here is on the physics of this and whether such a fissure would even stay open for any length of time. Obviously this is Sci-Fi and the laws of physics can be bent a little, especially with "advanced technology" as a helper, but I don't want to go too far beyond the bounds of reality.

Also, I'm not sure of what method would be used to create the fissure. In theory, if the humanoids had a weapon or tool capable of drilling through the crust of the planet, such a thing could be employed directly against the aliens, so there would have to be some circumstance or limitation to block that.

Question

So, any good ideas, or any comments or suggestions on my main idea to make it workable?

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closed as too broad by Mołot, L.Dutch, James, Hohmannfan, Azuaron Apr 3 '17 at 13:31

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ More backstory is needed. What motivates the water aliens? Are they intelligent and bent on conquest? Was there some misunderstanding and they perceive the land dwellers to be a threat? Why are they on the planet in the first place? If they are animals like wasps living in a hive with no tech, how did they get to a different planet? Are they the shock troops of a different group of aliens who dropped them off to soften up the natives? If so this different group must have a way to clear them out after the work is done. $\endgroup$ – Willk Apr 2 '17 at 18:46
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    $\begingroup$ Have you considering something similar to electrofishing to stun the aliens when they approach the coast? $\endgroup$ – L.Dutch Apr 2 '17 at 19:34
  • $\begingroup$ L. Dutch's approach seems like an excellent idea to me, large scale deployment of electricity from a space-faring civilization into the local body of water would increase temperatures also. But whilst we're at it, it could be used on the surface also, amphibious creatures are likely to be incredibly vulnerable to not only the raw electrical effect as w/ interrupting function, but also with ionizing atmosphere and increasing temp in an atmosphere they're not 100% comfortable with in the first place. Hardly think that answer I wrote was worth the words now :) $\endgroup$ – mensenisevirem Apr 2 '17 at 20:20
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    $\begingroup$ Don't use large nuclear-powered mechas. :P $\endgroup$ – ifly6 Apr 2 '17 at 20:36
  • $\begingroup$ Sounds a little like Harry Harrisons's novel (series) "Deathworld". $\endgroup$ – StephenG Apr 3 '17 at 0:19
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The humanoids can deploys a network of modified electrofishing boats all along the coast (I got inspiration from Electrofishing for Whales).

In this way the aliens approaching from underwater will be stun and left unable to proceed further.

Based on the logistic capabilities of your aliens, you may not need to protect all the coast range. To understand why, we can use a terrestrial example: in principle you can siege New York by landing troops on the Pacific coast in Oregon and then crossing coast to coast catching the new yorkers unprepared for this clever attack. But, as Hannibal, Napoleon and Hitler learned at their own expenses, long logistic chains are a nightmare and will on the long run lead your army to catastrophic defeats.

Also depending on the physical abilities of the amphibious alien, they may need to dive in water from time to time, and this limit the distance they can travel on dry land.

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  • $\begingroup$ Not bad, as mentioned before. I will check out the XKCD :D A thought based on your mention of the distance limit, perhaps they have a range of how far away they can get from the hive. If the humanoids can limit the travel of the slower-moving hive creatures, they could prevent spreading to other continents, if not up and down the coast. $\endgroup$ – Matt Mc Apr 4 '17 at 8:33
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If it is possible story-wise, one approach would be to use an attack vector that, for whatever reason, cannot be used against the main hive but can be used to destroy any invaders that travel too far away.

For example the alien hive could be in a cave system that protects them from surface attacks, but if they venture away from the cave system your aircraft can destroy them with simple depth charges.

Or, perhaps, there is some sort of undersea plant life producing copious quantities of oxygen or microbes that break down toxins, but that these are localized around the hive so the attacks you list would be effective against any invader that strays too far. Or there could be strong currents in the area that wash away pollution and replenish oxygen too quickly for those techniques to be effective, but the currents are only right around the hive and heading out to sea. Your approaches could even be used to create an wall around the hive that kills anything that tries to cross it.

There could be a localized magnetic anomaly that makes guidance systems for weapons too inaccurate or outright renders weapons ineffective right around the hive, but any invader too far away is easy pickings. This could be either naturally-occurring, the result of whatever brought the aliens to the planet, a leftover from some wreck or accident created by the natives, or even the result of a failed attempt to destroy the aliens.

It could be that the natives' weapons aren't very effective in deep water, but work very well in shallow water. The hive is situated in deep water right off the city, but gets shallow very quickly away from the hive. It could even have been intentionally dredged for shipping, or be naturally deep. Either way this sort of deeper water is needed for a shipping port, which many cities are built around.

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  • $\begingroup$ Wow. I like almost all of these suggestions. Your analysis that there needs to be an attack vector that cannot be used against the main hive but can destroy any invaders that travel too far away--spot on. I think the trick is picking out one of these ideas that's the most plausible, and perhaps that's my job, but I think the cave system or the magnetic anomaly are very workable. Hm... $\endgroup$ – Matt Mc Apr 4 '17 at 8:30
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Q's first:

This alien species/hive/invasion vector has a population limit?

Your description implies strong atavism, so I'm assuming either they are not using tech and/or have no access to it at all?

Does the species know that humans exist elsewhere on the planet? Does it know anything much at all about how people live, about surface geography?

Is it even a singular species and forma? The implication here seems that it is, but as we don't know how they got there (etc.)

How are they getting on to land? Ports aren't particularly shallow waters and the surface beneath tends to rise relatively suddenly (not too sudden mind, you'd get crazy surface currents then) The assumption then is that they're not 'walking' in the water, but rather are capable swimmers.

This matters because it defines their ability to flank and expand the battlefront, and your ability to exploit their mobility.

Redirection & Attrition:

Whilst the city must be fortified if you wish to save it, your best course would probably be to encourage them to flank the city by clearing channels and creating killing fields such that only those closest to the city get whacked with utmost efficiency, this will naturally create a flanking effect as any attacking mob is more likely to follow somewhere where it's allies aren't dying immediately, so long as it thinks it's gaining ground. So you have your port 'circle' made as impenetrable as you can make it, perhaps with airstrikes clearing out occasional waves to give defenders a breathing space etc, whilst the land to either side is defending by one fallback position after another, leading the creatures inland. Once the taken land is saturated with enemies you carpet bomb the area and reclaim the territory or not and repeat the process.

Killing all of them in one place is not to your advantage, everything more intelligent than an ant will then try alternative routes.

Even if your humans have been preparing for quite a while any large scale poisoning will be difficult. First you have to know what is poisonous to these creatures(?) Secondly the sheer quantity of the substance you'll need is massive, so unless this is an extended war unlikely to come about. Your poison needs to be heavier than water so it sinks and doesn't just get carried away by currents or kept at the surface by physics, or deployed with 'precision' attacks..Well, actually that wouldn't be so hard.

L. Dutch suggests using electrofishing techniques which seems like a good idea, granted I'm operating purely on intuition merely from the word itself, and I'm not aware of contemporary technology or deployment practices in this. But it seems to me that a civilization with 'spaceships' has significant power sources and the other required materials to produce stable devices which could continuously pump massive amounts of electricity into a body of water. Clearly(?) the devices would boil if they operated continuously at high voltage, but conceivably they could be designed to do this and be dropped in waves, or designed to pulse less regularly. If deployed akin to 'depth charges' the source of the enemy could be assaulted in the same fashion.

It seems to me that unless your enemies are particularly resistant to both electricity and extreme changes in local water temperature, this or a similar approach is best and rather wastes the words I put into the beginning of the post :)

Saying that, you're trying to make a story and story kinda requires jeopardy, so you might want to come up with some way of making this less than totally effective.

///The aphis.usda link from xkcd has some material (and further references) covering current in water

Also: https://cdn.intechopen.com/pdfs-wm/21295.pdf (Edo D'Agaro, U. of Udine) should prove useful.

Now clearly electro-fishing techniques seek to reduce damage to the catch, so read accordingly.

//I'm not suggesting that you'd want to electrify the entire body of water, sorry if I omitted the proposal of 'depth charge'-like devices. The idea is that you can deploy (air or sea vessel drop) heavier than water devices (scale is up to you) that deploy current from internal generators (or you could connect them via cables to surface generators) with or without sensors/timers etc et al.

I'm also assuming that these creatures attack native fauna, or at least said fauna will keep a bit of distance, tho I don't know much about sea-life behavioral patterns it has to be said :)

There are corollary effects on the water and chemical constituents also especially with prolonged use.

Of some interest might be direct application of fairly simple chemical compounds...anything that can debond the compounds in saltwater, dropped in capsules and exposed to the water (of a sudden) will produce effects much like a bomb dropped in the water, but without the cost of formulating expensive chemical explosives.

Earlier I suggested that poison deployment would require too much material..and whilst this is true for a direct-effect toxin, you don't need to directly affect the creatures concerned, and as they're attacking you along 'horde' paths the water instead can be targeted. Anything that affects the oxygen content of the water will of course affect any creature trying to breathe it. The duration of such affects will of course be negligible, with the vast quantities of water involved, but localized areas akin to surface mustard gas clouds could be created by deployment of capsules.

Which brings up hydrolysis and anions as weapons of war.

I also forgot to ask how far out this hive is. If it's a long way out, the depth of deployment means that these creatures aren't simply amphibious, but have a morphology capable of operating at very different pressure. Don't know if I need to go into the ramifications of this with you, but in order to be reasonable (and have your aliens be so) this ability will help to define their biology to some extent.

Attacking them at home specifically!

Waves/layers of depth charges dropped from aircraft/ships: I'd suggest dropping them in a cylindrical shape, with electrical types on the outside with irregular pulsing to stop the zerg from being ordered into suiciding onto the bombs in order to carry them off, or whatever, with an with the inside of the 'cylinder' composed of munitions to actually destroy their base of operations. Bear in mind that you probably don't actually need to 'hit' anything with such munitions (depending on how badass these aliens and their BBEG's are) the scale of disruption you're causing at the ocean floor and the local water there is going to be pretty massive and extended..and aside from all the kinetic activity, gonna make the water silty as all hell and etc.

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  • $\begingroup$ Whoa! Will try to answer e/g. No population limit. Aliens have no tech, think Zerg. Only a few members of the species have enough intelligence to know that they're in the act of conquering a planet; the rest are just hungry and follow mental orders. One primary forma, plus the hives and a third type that's very intelligent and sort of a general. They are very capable swimmers, think webbed feet. $\endgroup$ – Matt Mc Apr 4 '17 at 8:16
  • $\begingroup$ Thanks for your analysis of the basic combat tactics the humanoids would have to employ; pretty much spot-on! The electro-fishing technique seems fair, but I'm not really familiar with the physics of what happens when you pump tons of amps into open water. Wouldn't this require huge quantities of power to make something that would kill, even down to the ocean floor? $\endgroup$ – Matt Mc Apr 4 '17 at 8:18
  • $\begingroup$ Added some guff. Just saw your comment on other A and re-emphasizing problem of attacking them 'at home.' added more, sorry. $\endgroup$ – mensenisevirem Apr 4 '17 at 22:19
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It seems like your tech level is "near future," since you're talking about spaceships and energy weapons but they still need a seaside port for shipping goods. This implies that they probably have a fairly limited energy budget (since otherwise they'd just send everything via plane/spaceship).

The most straightforward idea for a simple barrier against sea bound monsters would be a series of propellers anchored to the bedrock pushing a strong current away from the shore. A unidirectional current like that could be difficult enough for individual creatures to overcome without completely nullifying the threat. Mind you, there'd still need to some circulation bringing water behind these blades, possibly creating a weakness to be exploited in the narrative.

Portions of the barrier could be shut down to allow passage of ships into the city. The outward current could also boost ships leaving the port past the "danger zone" at a higher speed.

Tl,Dr; a strong current away from shore created by spinning propellers of death should be a functional and efficient deterrent without completely eliminating the threat. It also creates interesting narrative opportunities.

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  • $\begingroup$ Perhaps "port city" was a misnomer; city-by-the-sea is more what I have in mind. But the humanoids do not have unending energy sources. As for the propeller idea, the humanoids are able to fend off the aliens through conventional warfare techniques, so the question is how to keep them from spreading out through the ocean, and I don't see how the propellers help with that. But thanks! $\endgroup$ – Matt Mc Apr 4 '17 at 8:20
  • $\begingroup$ You can encircle their localized habitat with these propellers to contain them. you'd need a broad circle, since the setup process is fairly slow and would leave the machinery and manpower vulnerable during construction. $\endgroup$ – Isaac Kotlicky Apr 4 '17 at 13:36
  • $\begingroup$ I wonder how much water disruption could be caused by the local deployment of the world's entire fleet of submersibles. /wonders how plausible it would be to attract tens of thousands of sharks and dolphins and etc to the local area.by 'dragging' with lures, would probably take months anyway..and they'd get distracted and swim off. $\endgroup$ – mensenisevirem Apr 4 '17 at 22:30
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Use Oil. Since it floats on water, it would not kill them, but prevents them to get out of it.
But if they have "oil-skin" that prevents body to get wet - use Detergents.
(New dish detergent, that can get rid of any water alien and protects your home port.)

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  • $\begingroup$ I think this is not very workable, and/or has crazy ramifications on the planet's ecosystem. $\endgroup$ – Matt Mc Apr 4 '17 at 8:30

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