I am aware of the square-cube law, so what conditions do I need for this to work ? The tortoise will be a similar size to the isle of Wight and will walk around in shallow areas in the ocean. What conditions do I need for this to be feasible without magic? I am aware of other questions about similar things such as oceanic creatures swimming underwater and turtles, but I believe this is different as tortoises do not swim fully submerged and are not turtles and would as I propose walk rather than swim. What I believe is the main question is what limits in size are there as the tortoise would be half an submerged walking with its head above the water.

Finally the question is what conditions do I need, not magical ,to have the tortoises proposed?


magic or an equal amount of handwavium

using real world physics its limbs would need to be bigger than its body to support its weight.

the important thing to remember is as you increase size the mass increases by the cube (the volume) while the strength of the bone only increase by the square (the cross sectional area). so you have to make the bones progressively thicker to withstand the forces.

You mentioned the square cube law but I don't think you grasp what it means. At a certain point the increase in bone creates more mass and thus requires more strength than the actual strength it creates.

And that is before you take into account things like surface area of the lungs, getting rid of body heat, or the blood pressure created by "water" column height.

Your best bet might be to not explain it and just make it your acceptable break from reality.

  • $\begingroup$ Yes I know all this as I said, but how could I make it work without magic? $\endgroup$ – Mendeleev Dec 26 '16 at 17:27
  • $\begingroup$ While I cannot comment on other issues right now, the surface area of a lung is almost fractal, and thus not usually an issue in scaling animals up. $\endgroup$ – Myrdden Wyllt Dec 26 '16 at 17:49
  • $\begingroup$ The issue with the lungs becomes the surface area of the lungs compared to the volume of the lungs, and the volume of air that needs to be moved, friction starts to exceed the forces the lungs can withstand/generate so you can't get fresh air into the lungs. $\endgroup$ – John Dec 26 '16 at 17:53
  • $\begingroup$ @Mendeleev you can't, a living thing has to many built in constraints, even if you use exotic materials or move the creature into microgravity you still run into problems of gas and nutrient exchange, and simple thing like body heat. An organism that size is generating heat faster than it can shed it just becasue it is tortoise shaped. There is just no way to do it using hard science. $\endgroup$ – John Dec 26 '16 at 17:59
  • $\begingroup$ @John it is not hard science just science based, and is there no possibility of low gravity or dense gases or heat absorbents making it possible? $\endgroup$ – Mendeleev Dec 26 '16 at 18:39

The turtle's bones are infused with an element far heavier and more dense than calcium and are made up of tiny triangles in a lattice pattern (perhaps crystalline?). Allowing the structure to support more weight with less mass. The turtle's blood has a high alcohol (or some other liquid) making it thinner and easier for the creature's heart to pump. This also has an "anti-freeze" effect, allowing it to more efficiently retain heat. Still, it sunbathes most of the day and moves very slowly due to its low metabolism.

  • $\begingroup$ What about other organs, would they work. $\endgroup$ – Mendeleev Dec 27 '16 at 15:16
  • $\begingroup$ I imagine they would. The biggest hurdles would be the circulatory system, lungs, liver, and kidneys. With the blood being thinner I believe this will solve that. $\endgroup$ – Colin Madden Dec 28 '16 at 4:57
  • $\begingroup$ Why would the density of calcium matter? The inorganic part of one is made from carbonated hydroxyapatite, and not pure calcium. It would also metabolize any alcohol in the blood fairly quickly, making it one huge, mean, drunk piece of turtle to run into. $\endgroup$ – Mrkvička Jan 19 '17 at 15:14

Since you only specified 'no magic', here's the sci-fi solution: Have a turtoise cyborg.

A 'thin' layer of organic matter (flesh, blood, ceratin, etc.) that covers the actual metal body. There's nothing else that would be able to support a multi-kilometer-sized body - no bone matter can be made strong enough to span such distances, no matter how much hard science you throw at them. Have the fusion / fission reactors cooled by the sea water, and you cover the problem of heat development. You will need some kind of blood, oxygen exchange, and glucose intake to keep the 'thin' (I'd expect a meter or two) organic layer alive, but that can be hidden within the metallic body.

And since you do not have to make the turtle cyborg interior solid, it should have some buyoancy, kind of like a ship. Maybe even enough to be able to support itself on the sea surface once it lifts its legs (which would be more like anchors only, not supporting any weight but tying the turtle cyborg ship down). A nice side effect would be that it can't just dive below the sea surface, washing away any and all vegetation that has started growing on its huge turtoise shell.

  • $\begingroup$ Thanks, this actually makes sense in my planned story so that is great. $\endgroup$ – Mendeleev Jan 14 '17 at 21:08
  • $\begingroup$ Once again you may of accidentally said turtles instead of tortoises. $\endgroup$ – Mendeleev Jan 14 '17 at 21:10
  • $\begingroup$ That’s pretty much what I had in mind, except it wasn’t explicitly artificial—it could be a reef-like colony being or alien. $\endgroup$ – JDługosz Jan 15 '17 at 9:45
  • $\begingroup$ @Mendeleev Sorry about that, I tend to use turtle and tortoise as synonyms since they are translated with the same word in my native language. Not sure how well it would go over to have a mile-long thing walk across solid land, especially since you're missing the water cooling there... but it should work for an artificial moving island :) $\endgroup$ – subrunner Jan 15 '17 at 16:15

Turtles, no. Big, living/growing, slightly-mobile floating islands, maybe.

I don't think you can get there with Turtles, per se. However,
if what you want are big/huge, living/growing islands -- that may be doable with only modest amounts of handwavium.

  1. Compound organisms, working together, can build structures that dwarf any individual member and survive many times the longest-lived individual. A key example are the polyps that create coral:

  2. Some seaweeds form substantial drifts or rafts, when the conditions are right, as in the Sargasso Sea: https://en.wikipedia.org/wiki/Sargasso_Sea#Ecology http://www.seaweed.ie/sargassum/sargasso.php

{With a minimum of handwavium, I conjure or} Posit a hybrid lifeform, like Euglena:
in that it can both photosynthesize, consume food, and move about in the water. Now, further posit that these have mutated, so that they:

(a) Create durable, buoyant structures (biological syntactic foam) as part of their bodies, much like diatoms -- but with cellulose walls enclosing methane.

(b) Form colonies, for mutual assistance staying in the sunlight. These colonies grow into hexagonal 'rafts' of up to 10-meter diameters, before specializing further. At the raft stage, they have only very limited feeding and mobility, but they do very well keeping afloat and in the sun.

(c) Successful rafts begin to differentiate, forming structural ribs to survive storms, and streamers (much like jellyfish of your nightmares) for improved feeding and mobility. Fully differentiated islands have been observed up to TBD kilometers. Xeno-genetic analysis of debris from two of the larger islands observed in the !@#$%^ survey indicated tissues up to 700 earth standard years old.

The thickened ribs surrounding the individual 'rafts' result in a turtle-shell appearance (and topography) of the islands. Durable, floating coral-like colonies -- except that they can feed themselves on unwary sea creatures that venture too close. Unclear whether they have defenses against surface predators....

I leave the rest of the creature design to you.
Have fun!

  • $\begingroup$ Great except that mentioned turtles, I specifically said tortoises but I find your idea helpful. $\endgroup$ – Mendeleev Jan 14 '17 at 21:09

It looks like a tortoise, but is built very differently. It’s mostly a hollow facade of tissue forming the floating mass. The legs have neutral boyancy and are more like anchoring cables: a larger apparent diameter is a protective sheath that creates a suitable environment for the “rope” but it is all hollow. To walk the anchor line crawls forward using a small benthic body serving the foot while allowing the line to slack, then pulls itself forward by tightening the line. The anchor lines are in tension, not in compression like normal legs we know.

  • $\begingroup$ So is it a natural creature? $\endgroup$ – Mendeleev Jan 14 '17 at 21:05
  • $\begingroup$ That works whether it’s natural or made. $\endgroup$ – JDługosz Jan 15 '17 at 3:43
  • $\begingroup$ Could you please explain what most of your terms mean I am not the smartest in this department. $\endgroup$ – Mendeleev Jan 15 '17 at 5:45
  • $\begingroup$ What terms are you specifically having trouble with? The general meaning of facade etc. should be easy to look up by right-clicking or holding your finger on any word. $\endgroup$ – JDługosz Jan 15 '17 at 9:44
  • $\begingroup$ Anyway, a hollow bouyant structure (like a ship or barge) seems to be what most of the answers are suggesting (everything posted after this one). $\endgroup$ – JDługosz Jan 15 '17 at 9:49

You want a giant tortoise that is screwed over by the Square Cube law if I understood your question correctly? Well I have numerous propositions for your.

No. 1: The turtles live in an environment with enough Oxygen to sustain gargantuan-sized life forms. The bigger an organism gets, the harder the heart needs to pump in order to get oxygen to every cell in the body (if I am not mistaken). So maybe you could increase the amount of Oxygen within the water, Or you could say that their hearts are more efficient at pumping oxygen throughout their bodies (or you can handwave and say that they don't need oxygen)

No. 2: The Turtles are nanotechnologically enhanced. Okay here me out on this one. WIthin the Nanotech community there's a thing called a Respirocyte. A Respirocyte is essentially a artificial blood cell which can store up to 236 times more Oxygen than a regular blood cell can. https://en.wikipedia.org/wiki/Respirocyte



No. 3: The turtles are robots meant to confuse biologists This one is pretty much self explanatory.

Anyway I hope you enjoyed my answer!


Bones and lungs

In order to support such a large and heavy animal, you'll need enormous and strong bones. Calcium bones would not be strong enough so you'll need something else, probably carbon fiber.

The inner structure of the bones will probably be hollow to reduce weight and latticed to increase strength. You can use this hollow space to house all or part of the respiratory system.

Additionally, this allows you to have a breathing system similar to that of birds, where air is constantly flowing in a single direction, this solves a potential issue that in-out breathing might cause, stagnation.

This kills a few birds with a single stone:

  • You solve the issue of getting oxygen to the entire body since now you can have the tortoise breathe from multiple parts.

  • You solve the problem of managing internal heat by having the bones act as a cooling mechanism powered by the air that is breathed.

  • You use most of the otherwise empty space that the bones will generate

  • You solve the whole "breathing" issue

  • The air-filled bones help the animal stay afloat in water.

Circulatory system

You'll probably need more than one heart to pump the blood of such a large creature. The blood itself probably accounts for a large fraction of the weight of the animal, so it's gonna be a difficult task.

One idea I can think of is, instead of having a distinct heart or hearts, make the blood vessels able to pump blood themselves. Having regular one-way valves in the tubes, and constricting muscles around them will probably work, the blood vessels will pulsate in a sequence that will move the blood around.

This distributes the workload to the entire system and solves the slight tendency of water to create vacuums above the 10m mark.


This is probably the most difficult part, moving such a large creature. Muscles will probably make up most of the weight of the tortoise just because of the sheer amount of strength needed to move it. Being partly submerged will help ease the load of actually having to hold up the animal, but it makes moving much more difficult since water is so dense and viscous compared to air.


Have it eat algae. Even better is if you have it grow its own food. Algae need water and sunlight to grow, the tortoise has both of those covered: it lives in the water and has a huge solar collector on its back.

The shell would act as a solar power plant AND rainwater collector to culture the algae, it can then slowly eat this food, just slow enough to allow it to regenerate. Whatever biomass the algae on its back cannot provide, can then be eaten the good ol' way.

The shell

I already mentioned the shell a lot. The way I see it, it would have to be made up of hollow interconnected cells. The outer layers would need to be at least partly transparent or translucent to allow the culturing of algae in these cells. The shell would also need to have a series of pores, some connected to the cells where algae are grown and others connected to the bones of the tortoise that would be used to draw in air.

On the inside, the shell would need to be supported by rib-like bones for both structural integrity and to be able to draw in the air needed to breathe. The inside surface of the shell would probably be very porous to allow the body to consume the food and resources stored inside.

From your other question, I believe you want to put people on top of the tortoise. The people could exploit these systems to keep clean rainwater. Algae are nutritious, so they also have a food source (just don't overexploit it)

Other organs

There are too many organs in an animal to feasibly list in this answer. Though I believe most of them can be distributed into a number of smaller organs, even the brain can be distributed through the body.

A few other details

Given the structure proposed and the way things would work, it will probably be best if the tortoise is mostly flat, that is, only a couple dozen meters high at most. This way you'll have a lot of shell area, which is very important here, very close to most of the body. It also eases on the load of having a lot of organs on top of each other, crushing each other.

It also makes the floating more stable and the structural load lower.


If the tortoise actually has to walk on legs, this is not going to be possible without magic or sophisticated technology indistinguishable from magic. If the tortoise doesn't actually have to walk, and most of its weight can be supported by the water, you might be able to pull this off by having it use its legs like barge poles to move around in the water. Also, it's not going to look much like a tortoise. Its head and legs will be very tiny compared to its body, which will contain a lot of empty space to produce the required buoyancy. It will probably resemble a giant engorged tick covered with debris.


If you handwave energy requirements, an underwater creature can be arbitrarily large since the density of organic materials are basically the same as that of water. The hard part to explain will be the part of the body sticking out of the water, in our case, the "shell". However, it could be that the shell is made of a buoyant material, like wood, or one full of air pockets, similar to pumice, allowing it to float. Pumice does eventually sink, but the shell could also be covered with a thin membrane that traps gas and some organic process could replenish it as it leaks. Decaying organic matter is a good source of gas. It doesn't smell too nice, but being pleasant to live on wasn't one of the requirements, was it?

The main issue with such a big creature is figuring out what it eats. If the shell is covered in plants it might have some sort of symbiotic relationship with them allowing it to gain energy from photosynthesis. It might also have something in it that attracts plankton or small fish into its mouth, allowing it to feed with minimal energy loss. Even so, don't expect it to be moving around very much.

Oxygen exchange will be an issue as well. This creature will not be able to raise its head above the water, so it will probably either be a water breather or breathe through openings in its porous shell. If it has gills, they may be extended into the surrounding water to make oxygen exchange easier, similar to the gills of an axolotl, but even bigger. This will make it more vulnerable to parasites, though.

There's not much reason for a creature like this to look like a tortoise, so if it's a natural creature I wouldn't expect it to. A giant jellyfish or kelp would be more likely.

  • $\begingroup$ Why would it need to float if it walks. $\endgroup$ – Mendeleev Jan 15 '17 at 11:32
  • $\begingroup$ @Mendeleev If it stayed entirely submerged, it could walk or swim fine; buoyancy makes weight irrelevant so the square-cube law doesn't really apply to aquatic animals (as far as support is concerned anyway; energy and oxygen are still an issue). But if we have a turtle island situation where part of the body was held above the water, there would need to be some way for it to support that weight. The most logical way to support a large mass above the water level is to make it float, otherwise we'd be dealing with the same issues that make giant animals implausible on land. $\endgroup$ – IndigoFenix Jan 16 '17 at 7:07

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