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I'm writing a story that takes place entirely underwater, mostly in a deep sea town and shallower city. Both are underwater and populated by beings who can survive underwater.

My thinking is that they send messages to each other by taking a soft clay-like material (let's call it substance x until someone thinks of what it should be) and writing on it, then giving it to a messenger. The messenger takes this wad of substance x with the writing on it and delivers it to the recipient.

When they are done the wad is given back, the letters are smushed away, and it is reused. What material should this be?

It needs to be something that can be made of substances found in the ocean, can be made underwater, doesn't dissolve into the saltwater, lasts a fairly long time, and is inexpensive.

Is there a real-world substance that meets all these requirements, or am I gonna have to make something up?

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What you described is an old invention:the wax tablet.

Spermaceti from sperm whales is just one of many waxes your underwater people could use. Some of the fishes also contain significant quantities of wax. If you need only a short-term solution, animal fat would also do the job.

I couldn't find any seaweed that secretes waxes, so I hope your underwater people are not vegan.

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  • $\begingroup$ Agh, I was just about to hit "post" on that very idea $\endgroup$
    – Starfish Prime
    May 23, 2019 at 21:47
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    $\begingroup$ @StarfishPrime have a +1 from me :) $\endgroup$
    – Mołot
    May 23, 2019 at 21:49
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    $\begingroup$ +1 for correct use of fishes. $\endgroup$
    – user1717828
    May 24, 2019 at 17:05
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    $\begingroup$ @JoeBloggs, Yes, you can use sheeps in both academic and colloquial settings. If anyone questions it, you have my permission to rely fully on the authority of internet citizen user1717828. $\endgroup$
    – user1717828
    May 24, 2019 at 19:36
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    $\begingroup$ A wax tablet is called a tabula, and the writing tool is a stylus. There's a good chapter about the tabula in Writing on the Wall. $\endgroup$
    – Walter Mitty
    May 25, 2019 at 2:07
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Braids of seaweed.

The ancient Inca used knotted fibers, or quipu, to record events and information. With enough effort, seaweed could be manipulated (slicing then braiding) into thin cords, which could then function in a similar way as quipu. I wouldn't be sure about the lifespan of these cords -- it depends on the manufacturing process -- but these makeshift quipu could have knots tied or untied, and would be relatively light or even buoyant in water.

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    $\begingroup$ Not dumb. I was think of knotted seaweed myself but didn't know how to make it work. I didn't know about quipu and appreciate the chance to learn about it. Welcome to Worldbuilding.SE. $\endgroup$
    – Cyn
    May 24, 2019 at 4:49
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    $\begingroup$ Quipus could not only record numbers on a string. They could also convey what IT people would call a tree structure, by attaching substrings to strings. The quipucamayoq, expert in this, could convey very detailed and subtle info. If you have this, you almost don't need an alphabet. $\endgroup$
    – Walter Mitty
    May 25, 2019 at 10:49
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You don't have to go back in time very far to reach a point where marine-derived hydrocarbons pretty much made the world go round. The catch being that you have to kill rather intelligent, large and dangerous predators who share your environment.

Enter the sperm whale, whose head is filled with an extremely useful collection of waxes and oils which may be extracted and refined to produce exactly the sort of material you'd be interested in. By way of a bonus, it also produces another kind of oily wax, ambergris. This is much rarer, so you would only use it for special occasions, but it would still suffice.

In both cases, you want to keep your message tablets safe in a heavy container or framework, as the waxes would be lighter than water and quickly lost if dropped.

Waxy molecules are found in various marine plants and animals in smaller quantities, so you don't have to wave your hands too hard to invent something slightly more easily and ethically sourceable. The main waxy component of spermaceti, cetyl palmitate is found in some corals. I'm sure you could conjure up a wax coral or sponge that produced it in usefully harvest-able quantities

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If you want a pliable material that can be collected undersea without having to kill anything, you might want to consider a bituminous clay or asphalt-like material formed in the vicinity of an undersea asphalt volcano.

High viscosity hydrocarbon fluids mixed with fine silt or sand produce an oil-based clay-like substance that can be scraped or carved to form letters. The marks will last a long time as long as the temperature remains below a certain point, but can easily be erased by simply heating the surface via friction. Rapidly rubbing the surface softens and smooths the markings.

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  • $\begingroup$ Clay is not water-soluble, especially if it's oil based, +1 $\endgroup$
    – Mazura
    May 24, 2019 at 19:00
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    $\begingroup$ I can see "heating via friction" being awkward for what would appear to be a minimally technological underwater society... $\endgroup$
    – Starfish Prime
    May 25, 2019 at 9:21
  • $\begingroup$ I specifically mentioned heating via friction because it does not require any advanced technology and can be done underwater, unlike other options such as fire or solar heating. $\endgroup$
    – barbecue
    May 25, 2019 at 18:36
  • $\begingroup$ @barbecue: The water is going to carry away the thermal energy resulting from friction very quickly without much increase in temperature of the "tablet" $\endgroup$
    – Ben Voigt
    May 25, 2019 at 21:49
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    $\begingroup$ @BenVoigt I don't think you fully appreciate the physics involved. The friction occurs at the point of contact between two surfaces. There is very little water in between the two surfaces, and the small amount that may be there is easily heated. The fact that the heat may be rapidly dissipated by the water once you separate the surfaces is irrelevant, because the softening and reshaping of the surface has already occurred at that point. $\endgroup$
    – barbecue
    May 26, 2019 at 16:09
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Maybe not 100% what you’re looking for, but you could use the bones of aquatic animals to carve into (I’m thinking a runic alphabet like Ogham, which was designed to be notched into materials like wood). Then, when you’re done with the message, you can file the surface smooth and start again. Obviously this will wear out the bones over time, but it might work as a temporary solution (and fairly easy to implement, as bones are probably going to be relatively abundant in a decently-populated underwater civilisation).

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I like that we are trying to find a realistic substance to suit some wonderfully unrealistic characters! I think the sperm whale may be the winner but my immediate thought was putty; the smooshy stuff they used use to put window panes in place. It's sort of fawn colored and can be scribbled on. Its waterproof for sure, to keep the rain out but fully underwater, I don't know...Good luck, I love the idea of your underwater community :) (If you are stuck, maybe they can blow bubbles in the manner of old smoke signals?! )

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    $\begingroup$ Hmm... how would we derive putty from underwater substances? The question asks for substances found underwater =) $\endgroup$
    – Cloudy7
    May 24, 2019 at 15:14
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Why does the substance have to be soft? Scraping on a rock (slate) with another rock (chalk) would work as well underwater as it does above.

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    $\begingroup$ Chalk washes off a little too easily. $\endgroup$
    – Mark
    May 24, 2019 at 21:43
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I'll answer your question very generic, so you can use it not only for writing tablets, but ANYTHING in your underwater world.

What you are looking for are hydrophobic substances such as oil, fat or wax.

For your writing you'll want fat-derived substances, given that wax will be too hard in the cold. But simply take a light granite tablet and slather some thickened, darkened grease on it. By "writing" on it you get light letters on a dark surface, and with your fingers or a simple ruler you can "delete" writing as well instantly.

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If your race as access to metals, and if they are stronger than humans, gold would be the perfect match. Especially if they're somewhat advanced, as there's a lot of gold floating in the oceans. The gold could be filtered out they would have a more or less infinite source.

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  • $\begingroup$ I suspect if they were technologically advanced enough to extract macroscopic amounts of gold from seawater, they wouldn't be looking for a wax tablet substitute. $\endgroup$
    – Starfish Prime
    May 25, 2019 at 9:22
  • $\begingroup$ They are not that technologically advanced. $\endgroup$
    – John Lewis
    May 31, 2019 at 23:07
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Fish Carve the message into a fish, the recipient eats the fish and sends back another fish with reply carved into it.

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    $\begingroup$ I'm pretty certain that dead fish will fail the "lasts a fairly long time" requirement. Also, writing legibly on a fish by carving is not straightfoward (I invite you to try; please supply pictures) especially if you want to write fairly densely. $\endgroup$
    – Starfish Prime
    May 24, 2019 at 12:53
  • $\begingroup$ @StarfishPrime What about a big fish? Like a whale $\endgroup$
    – Hugo
    May 24, 2019 at 14:07
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    $\begingroup$ @Halhex no matter how big the whale is, it still isn't a fish. $\endgroup$
    – User1000547
    May 24, 2019 at 14:33

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