In the movie Pacific Rim, we see the beginnings of an alien invasion using very large mobile lifeforms.

Given that we want as large a lifeform as possible with the following traits:

  1. It must be able to move under its own power at a minimum top speed of 45 kph for at least 2 minutes before needing to rest (reduce to indefinite duration speed). I.e. capable of outrunning a human on foot at maximum speed.

  2. It must be able to move under its own power at 10 kph for a minimum of 48 hours, i.e. capable of outrunning a human on foot over the course of this time.

  3. It must be able to live and function on the land surface of Earth for at least 14 days (absent hostile action) at full effectiveness. Bonus for being able to swim.

  4. It must be grown to its operational size over any time period of 25 years or less, not constructed, though its first building block/cell/functional unit may be constructed/engineered. It may be gestated in one of its own kind as a parent or in an artificial womb, but must do the work of growing itself, provided with only raw materials and energy.

  5. It must be capable of (and inclined to) inflicting significant damage to human infrastructure and populations, using non-biochemically-specific means (i.e. no toxins) over its full effective operational duration. If it is armed with projectile/incendiary weapons, it must be either equipped with sufficient magazine capacity to last 14 days of combat or be capable of in-the-field replenishment using found resources.

  6. Evolution is not a concern. These are created beings, so as long as they can be grown from a single unit, they can incorporate design elements not available to merely evolved creatures.

  7. It must be resistant to conventional (i.e. non-nuclear) human weapons of the current era (C2016), and must be able to absorb any practical amount of fire from standard man-portable firearms, and at least one direct hit from a tank-based weapon or at least one loadout of any conventional ordinance from any combat aircraft and still remain at least partially combat effective.

How big a lifeform could we realistically have, and in very general terms, what chemistry and body plan would it need to fulfil all of these criteria, if they can all be fulfilled? How (if at all) would it obtain additional energy for continued operations? Are Pacific Rim's kaiju realistic?

Preference will be given to answers that can justify greater combat capability, longer operational durations, and a shorter production time for a given magnitude of body mass and volume.


Think out of the box here, guys. Don't feel constrained to CHO chemistry or vertebrate body plans. If some sort of Von Neumann nanotech made from metal and powered by atomic reactors can be justified, run with it. Should the body resemble a starfish or a centipede or a sea urchin or something else?

  • $\begingroup$ a titanosaurus would not fit your requirements? Anyway, any Hollywood-Dinosaur is bullet-proof, so why do not browse this list en.wikipedia.org/wiki/Dinosaur_size and add some genetically engineered bone plates? $\endgroup$ Commented Jan 8, 2016 at 9:21
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    $\begingroup$ Your question seems very broad, too broad to be answered concisely, I suggest you split your question into specific questions that can be answered more objectively to ensure high quality answers. $\endgroup$ Commented Jan 8, 2016 at 13:13
  • $\begingroup$ I agree with @ScottDowney - you'll end up with answers only addressing why Pacific Rim is totally impossible rather than designing a creature like you seem to want. $\endgroup$ Commented Jan 8, 2016 at 16:30
  • $\begingroup$ I agree that you will get better answers if you split this question into other questions. Having said that, with a little modification to this question you could leave nearly all of what you wrote but ask one, more specific question here (so maybe @ the end ask 'How large could these creatures be?'). Then, really, you could do the same for your other questions (how to make them combat effective offensively, defensively, what would they look like, etc...) $\endgroup$
    – MER
    Commented Jan 8, 2016 at 18:27
  • $\begingroup$ Guys, this question is about engineering a big combat capable critter. It's a whole package, not a collection of separate questions. One factor affects the others. If you're designing a warship, you can't just whack together a hull and some weapons and get as effective a vessel as if it was designed as a whole. $\endgroup$
    – Monty Wild
    Commented Jan 8, 2016 at 22:52

4 Answers 4


It's all determined by gravity.

I'm currently writing a paper on a similar topic, observing how gravity affects the evolution of extraterrestrial life (size and internal build). Therefore, I can offer you some hints on how a mobile animal is limited on Earth, but I'm afraid this matter is complex enough without introducing the problem of how much damage the creature could withstand, or how it should be engineered.

First of all, the maximum mass an organism composed of the same muscle and bone as the rest of the animal kingdom can obtain was estimated by Hokkanen to be around $10^5$-$10^6$ kg. If you pushed nature to the limits like this, the required energy just to sustain this organism would be huge: by Kleiber's law, metabolic energy scales as mass to the power of $3/4$.

Say we neglect the huge energy required to keep this being alive for the moment, and want to find the energy required for locomotion. Every living being on Earth walks utilizing the inverted pendulum gait. By using the leg's natural period and expanding on the work of the hyperlinked paper, after some basic calculation, we've found that the work (energy) required for one step is

$W = mgL \left( 1-\sqrt{1-\frac{\pi^2}{6}F} \right) $,

where m is leg mass, L its length, and F is Froude number, which characterises the animal's gait and is equal to $v^2/gL$. The leg is 18% of a human's mass, let's say it's 30% of your animal's mass ($10^5$ kg), to be able to support its weight. For a 180 meter tall Kaiju, we can call 40% of that to be leg length. If you want it to walk at 10 kmph, this comes to nearly 20,000 Joules per step. (By comparison, a human requires 150 Joules per step.) If you want it to run in short bursts, the required energy would at the very least double, as a human's running energy approximately doubles compared to walking.

If you want it to walk for 48 hours, that's some 150,000 steps at this speed, which brings us to a total of $3 \times 10^9 J$. When you turn that into dietary calories, you get "71 times the caloric energy consumed by Michael Phelps per day when training for the Olympics", or "310 times the recommended food energy per day for an average person".

So our monster will expend, by Kleiber's law, roughly 180 times the human basal metabolic rate to stay alive, and with the expenditure of walking, this comes up to $5 \times 10^9 J$. Turns out a human cadaver has about 81,500 Calories of energy, so by feeding exclusively on humans, the monster would have to consume over fifteen thousand people per day to survive.

This would explain Godzilla's raging hunger, I suppose.

Obviously, I am a physicist and not a biologist, so my overly-specific solution only applies to spherical cows in vacuum. We could wildly speculate about other factors, dimensional analysis, allometry, science fiction which would loosen or impose additional restrictions to our monster, but by considering only its motion in the gravitational field, we can obtain this interesting play with numbers.

EDIT: If we wish to discuss completely artificial constructs, then the most efficient means of locomotion would definitely be rolling. That would involve no 'bobbing' of the center of mass, as happens when you walk, and thus completely relax the gravitational limit on size. However, a creature that's completely spherical or rolls on caterpillar tracks or crawls like a serpent would be severely limited in terms of available limbs, so that's not too effective for an (intelligent) weapon of destruction. Any other limb-assisted locomotion is performed by the inverted-pendulum model and the above calculation applies.

Additionally, were the creature to run on nuclear power, it could do so with a relatively small reactor - a power plant in the USA generates 10,000 times more energy per day than our monster would require. The energy it requires for basic functions is really not that large, it comes down to about the yearly expenditure of a clothes dryer, except in a day. So the only thing to consider is a very quick and efficient source of energy - consuming food is obviously one, albeit in large amounts, but a nuclear reactor would also do the trick.

  • $\begingroup$ The stuff on stride energtics is interesting. You seem to be assuming CHO chemistry and a vertebrate body plan, though. What if the creature had an atomic reactor or three? What if it was shaped like a snake and could move by rolling itself into a loop? $\endgroup$
    – Monty Wild
    Commented Jan 8, 2016 at 22:40
  • $\begingroup$ I'll add a bit on alternate power sources and means of locomotion, but honestly, the more you widen your question, the less accurately we can determine the maximum limit on any kind of size and energetic expenditure. We can only speculate and compare without any calculation possible, because if you assume we can engineer anything, there are too many variables to consider. I understand you'd like a bit of both, to discuss a variety of factors and to have a concrete numerical limit at the end, but I'm afraid it'll have to be one or the other... :/ $\endgroup$ Commented Jan 9, 2016 at 15:06
  • $\begingroup$ Any chance you could add a reference to the paper once it is published, assuming it is? $\endgroup$
    – user
    Commented Apr 26, 2016 at 14:01
  • $\begingroup$ Why not rolling tread AND arms? Make it like a Mark V, but instead of cannons on the sponsons, put arms instead. $\endgroup$
    – anonymouse
    Commented Oct 24, 2016 at 6:58

It's not precisely about gravity as much as it is about height. The largest natural creatures are likely as big as natural creatures with traditional body plans can be, but the constraints on an engineered kaiju are extraordinarily relaxed.

At all times a titanosaur must be mobile and competitive with other organisms, but a kaiju can be grown with arbitrary amounts of scaffolding etc etc. The 25 year time limit is plenty of time, considering that biomass scales exponentially. A 'kaiju' consisting of just bacteria fed as much food they want in the right amounts for 25 years has extremely large mass. (An E. coli has about a one hour doubling time on minimal media, and there are 219000 hours in 25 years. This means after 25 years there'll be about 4*10^65000 bacteria, which each weigh about a picogram. The high-end estimates for the mass of the observable universe are 4*10^70 picograms. 25 years for a gestation time is plenty.)

Consider exotic 'kaiju' which are just 500 tons of ants all joined together with little living whiskers in a giant creepy carpet. It's clear this superorganism can become nearly arbitrarily large. See Pando, who is heavier than Godzilla (much heavier) but shorter and spread out. Oxygen supply isn't a concern if you let yourself have multiple pairs of lungs or passive convective ventilation like a skyscraper(or you're a tree).

Weight is not a concern if you let yourself spread out horizontally. Respiration is not a concern if you let yourself be porous. The highest strength materials(by weight) are Kevlar or Dyneema or what have you. Admittedly these are tensile not compressive strengths, but if you have a tube that's full of water and about 50% water by weight you can convert one into the other using hydrostatic compression at a 50% strength penalty(blah blah laplace's law blah blah optimal radius blah. We're exceptional engineers, we'll figure it out). Anyway. Concrete is one of the weakest construction materials by specific strength, and cannot get more than 400 meters high under its own weight. A concrete post 500 meters high will crumble even if it is prevented from snapping or buckling. Even if we stick to traditional organic materials (spider silk) we can build a post 100km tall if it's prevented from snapping or buckling. Beings built from concrete can really never be more than 400 meters high unless you build pyramids.

So we want to build a giant creature that can break things. That means we want arms or legs or something, so we have to stay well below our maximum height so we can have non-load bearing space for brains and lungs and eyes. Additionally a big club might take much much more strain while being swung than the club would experience under gravity. We can estimate the maximum permissible height for realistic walking-around loads using titanosaurs. Titanosaurs were about 50m high, and used bone and flesh. Bone's self-supporting length is about 20km plus minus, so walking and being a real creature means the skeleton has to support about 400 times gravity stresses. Dyneema can support itself up to almost 400km, so using this extremely dirty heuristic a Dyneema-skeletoned creature could be almost 1km high and still stand up and theoretically walk around. If it was dinosaur-shaped it would have Other Problems(like breathing, and the poor maximum strength geometry of muscles) but the principle is "sound".

Building a giant starfish about 30 stories tall and starfish-shaped would definitely be able to destroy buildings but I'm not sure about the speed requirements. Anyway. An organism on this scale is immune to conventional small arms fire essentially trivially, as you can just write off the outside meter of flesh as expendable and then you have 100 cm of ablative armor. Technically a lot of automatic weapons could act like a sandblaster(eventually) but I don't know who has that kind of time/ammunition. Anti-tank rounds are actually easier to defeat than conventional weapons, because they are so specialized for thick layers of metal. Very thick layers of flesh(from the point of view of a tank-sized object) dissipate the energy well before it can reach anything important, and conventional high explosives can be slowly dissipated by a composite ceramic/kevlar (or shell/cartilage, depending how you look at it) layer.

tl;dr Being tall is hard, being big is easier than being tall, the hardest part about this is going fast while being big or tall.

  • $\begingroup$ Anti armour rounds would punch long, narrow and very deep wound channels into the flesh of such a being (both HEAT and APDSFS rounds), which should have a negative effect, especially since they would have a better chance of cutting through structures like blood vessels, nerves, tendons and so on (a mechanical being would see damaged cooling lines, power and communications cables and mechanical transmission elements). Even a 105mm APDSFS round hits with 13 million foot pounds of energy, which will really make someone's day. $\endgroup$
    – Thucydides
    Commented Jan 9, 2016 at 4:47
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    $\begingroup$ The penetration depth of a liquid projectile like anti-armour rounds is (nearly) strictly density-based. At 17 km/s, even steel flows like water. See en.wikipedia.org/wiki/Impact_depth for mathematics and assumptions. Notably the penetration depth is limited to about 3x the length, or for HEAT rounds the surface-charge distance at detonation. Flesh with a high water content would function like reactive armor due to steam explosions, but that effect won't help a great deal. 10 meters of -anything- is just really hard to penetrate. $\endgroup$
    – Resonating
    Commented Jan 11, 2016 at 15:41

Make the kaiju a eusocial clonal colony swarm.

As many answers have already mentioned, scaling laws make the weight and energy costs of large animals prohibitively high. However, we may be able to draw from other kinds of animals to reach larger final sizes of the kaiju.

Insects are surprisingly fast, even without correcting for their body sizes. This BBC article cites this paper which measures beetles moving at 9km/h.

Similarly, American cockroaches were timed at 5.5km/h while walking. They are also very capable of flying, and a swarm of flying sub-kaiju can easily reach the top speed required.

With additional augmentations, these individualised organisms can easily move at the maximum walking speed of a human for long periods of time.

When not actively attacking a city, the sub-kaiju would maintain a colonial structure similar to that of Dictyostelium slime molds, maintaining contact as a swarm but not actively linked to each other. In this form, they can also forage and act as locust swarms, destroying crops as well as the local economy.

However, when they need to attack a city, they can join up into a larger structure (much like Dictyostelium fruiting bodies) which is capable of dealing damage physically to buildings. The combined colonial kaiju should be able to perform tasks similar to the microbots in the movie Big Hero 6

This mode allows them to easily replenish their energy levels and reproduce (by living like terrestrial cockroaches). They can spread into swarms and raid the depopulated cities, converting any destroyed sub-kaiju, killed humans/animals and food from abandoned supermarkets into more biomass.

Furthermore, their colonial structure renders them highly resistant to conventional attacks. By simply de-linking small fragments of themselves, they can effectively resist any physical weapon such as guns, bombs and artillery. In fact, the extreme hardiness of insects (as demonstrated by cockroaches' resistance to nuclear bombs) means that the combined colonial kaiju can simply delink to survive even nuclear explosions.

  • $\begingroup$ Not so sure about the nuclear explosion part. Each individual unit of the swarm creature will be absorbing a lot of energy in the form of ionizing radiation, high energy photons as well as physical blast effects. Most of the units will be too damaged to come together again and swarm. You might reduce a dinosaur sized creature to something the size of a small house pet once all the undamaged units found each other again. $\endgroup$
    – Thucydides
    Commented Jan 9, 2016 at 4:41
  • $\begingroup$ The "survive nuclear explosion" is misquoted. They can survive the radiation better than humans in the aftermath of a nuclear apocalypse, but the explosion itself will kill them just fine. Also, there are other insects that will beat the cockroaches. $\endgroup$
    – Nelson
    Commented Mar 4, 2020 at 2:19

The short answer is No.

The long answer is that the way biology has evolved makes it impossible for creatures to grow much larger than they already are.

Bones can only be so strong, support so much weight. A heart can only pump so much blood, and tissue can only be so "resistant" to damage - and no living creature could take an armor piercing tank round and live - there's simply no tissue that could withstand that.

Modern Animals

Blue Whales, which are the largest animals on Earth, suffer from back injuries as they get older, simply because of their large size (this is even though their bodies are suspended in water their whole lives).


Dinosaurs existed over a very large period of time. The closer we get to present day, the smaller they got, however. As they evolved, nature found smaller designs to be more efficient.

The largest of them to ever exist (so called Titanosaurs) were 20m tall, and as long as 40m. These were herbivores, moved very slowly, and are believed to have had multiple hearts in order to get blood from one end of their bodies to the other.

While that's impressive, please remember that they didn't withstand the test of time, and also that they are the absolute biggest that anything ever got on Earth. No Godzilla like creature could possibly exist - it would pancake under its own weight.

In Science Fiction

If you look at movies such as Avatar they explain the biology of Pandoran creature (massive flying dinosaur type creatures, and incredibly tall humanoids) as a byproduct of their low gravity and naturally occurring Carbon Nano-tube in their bones, which make them "very hard to kill".

Even carbon nano-tube wouldn't be able to help Godzilla out though.

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    $\begingroup$ I don't think the OP wants to let them loose, wait a few hundred years and see if they are still around. I understood it like they want to use them as weapons of mass-destruction, like drop in NY, wait a week and conquer the remains with ease. $\endgroup$
    – JFBM
    Commented Jan 8, 2016 at 14:46
  • $\begingroup$ @J_F_B_M - and I just said that those creatures (Godzilla like) couldn't exist for one minute, let alone a week. A titanosaur could exist (it has in the past), maybe even be genegeneered to be twice the size by using carbon nano-tube in its bones, etc, but it would still be quite easy to kill with anything larger than a hunting rifle (.50 Cal sniper rifle/machine gun would suffice, never mind a tank AP round, or an air strike). These creatures would not be major threats to buildings, etc like the OP wants. $\endgroup$
    – AndreiROM
    Commented Jan 8, 2016 at 15:45
  • $\begingroup$ which does not answer the question. I understand that Godzilla can't exist (though you should mention the thing with growth of strength vs growth of mass). The fact with anything larger than a hunting rifle is correct, but is not in the answer. To me your answer still sounds like you are expecting some evolution, but nobody cares if the creature has backpains in two weeks. It is all engineered. EDIT: Ok, I'm more ok with your edit now, which apparently happened while I typed. $\endgroup$
    – JFBM
    Commented Jan 8, 2016 at 15:55
  • $\begingroup$ Might want to make it clear that you seem to only be answering the last question, which is, "Are Pacific Rim's Kaiju realistic?" - otherwise just about all your points except the one about tissue not surviving an armor piercing tank round don't apply to what I would consider to be the "actual" question - "How big a lifeform could we realistically [bio-engineer]?" $\endgroup$ Commented Jan 8, 2016 at 16:25
  • $\begingroup$ I've removed the bit about "Are Pacific Rim's kaiju realistic?". As @DoubleDouble said, you're stuck considering evolution, CHO biochemistry and vertebrate body plans. I don't care what these critters are made from, or if they look like a dinosaur or your worst nightmare as long as they meet the stated criteria. $\endgroup$
    – Monty Wild
    Commented Jan 8, 2016 at 22:58

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