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Many questions on this site mention how difficult it is for merfolk to develop technologically, as there's almost nothing but rocks and kelp to work with. My solution to this is that they use these rocks to build sea-spires that put them right in the center of maritime trade. These spires would essentially act as a port between continents for kingdoms lacking the economic might, navigational skills or shipbuilding know-how to sail all the way across the ocean. The towers would provide landmarks, trade, shore-leave, ship repair and other services on the jetties ringing the lighthouse like tower.

For what the tower is built of, materials acquired from the ocean or close to the shore are preferable. Merfolk can breath & walk, but to them it's like crawling is to us. This means they'll heavily prefer to trade for materials from further inland, making them more expensive and harder to acquire.

The deeper and wider the ocean, the more valuable these towers become but the harder they are to build. Can these towers get up and out to a useful height and depth or would the currents and weight topple them before they get out of site of shore?

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    $\begingroup$ Welcome to Worldbuilding.SE! We're glad you could join us! When you have a moment, please click here to learn more about our culture and take our tour. Roman concrete gets harder under sea. and the merefolk of an alternate-earth could have traded with ancient Romans for the tech. This doesn't answer your question, but it's food for thought. $\endgroup$ – JBH Mar 31 at 2:57
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    $\begingroup$ There is such a thing as underwater mortar, but the underlying problem is more that making any sort of cement or mortar or concrete requires a lot of heating and drying of materials will will be tricky for an aquatic species! $\endgroup$ – Starfish Prime Mar 31 at 7:03
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    $\begingroup$ "Mortar won't harden underwater:" oh really? Even the Romans had hydraulic mortar, two thousand years ago. $\endgroup$ – AlexP Mar 31 at 10:23
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    $\begingroup$ Huh, I didn't know of hydraulic mortars. The Roman concrete definitely seems like a good fit for this. Heating and drying can be done on-shore. I'll have to think on if it makes more sense for the merfolk to make it themselves or trade for it. The merfolk do have some internal magic that lets them breath air and walk, but this is starting to become a different question. $\endgroup$ – jb6330 Mar 31 at 13:34
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    $\begingroup$ Concrete and mortar are two very different things, used for very different purposes. The Romans had both hydraulic cement (used for making concrete) and hydraulic mortar. While the knowledge of hydraulic cement was lost for some time (until the 19th century), hydraulic mortar has never been lost and was made and used throughout the Middle Ages up to modern times. $\endgroup$ – AlexP Mar 31 at 15:11
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Currently, static oil rigs built on reinforced concrete towers are built in sea depths of up to 350 metres. This isn't the limit of the structural mechanics, but more of an economic limit.

The deeper concrete structures are built using concave towers (thicker at the bottom and the top). The thicker top actually provides some buoyancy reducing the force on the structure vertically and the slender middle reduces the horizontal force on the structure created by the ebb and flow of the oceans.

I very much doubt your Merfolk have the maths, materials and merman-power to create a concrete structure such as this.

But, the interesting thing about looking at oil rigs in regards to this question, is that the deeper the water, the simpler the design.

After concrete structures, you get into steel frame structures, these go up to 1 km in depth. Still not a material readily available to Merfolk and I doubt the could do the underwater welding required.

But next, comes the simplest design. Moored floating platforms.

So what is required for one of these?

  • A floating structure - and us humans have a habit of leaving lots of wooden boats at the bottom of the sea.
  • Heavy weights to sit on the bottom of the ocean - plenty of big stones in the sea.
  • Long enough ropes - this is probably the trickiest thing for the Merfolk to get hold of. But those shipwrecks would all have had anchors right? And those anchor ropes could be spliced together. You'd just need enough of them to keep your floating platform in roughly the same position

TL;DR - Don't build giant underwater towers, build a floating platform and anchor it in place.

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    $\begingroup$ Good points, but one correction. If you are out in the open sea and not sitting at some ridge, there are not many big stones there. The bottom is just a deep layer of sediments. (I have been at research ships taking deep water samples, if it comes up with a pebble, it is a small sensation and it means it has been dropped from an ice berg) $\endgroup$ – MortenSickel Apr 1 at 19:25
  • $\begingroup$ Floating platforms do seem to fit generally better. I'll have to do more searching to see how big/tall/impressive I could make those and will definitely use them in deeper water. I'm still kind of in love with the idea of a solid tower rising from the sea, but the 350 meters gives me a limit for where I need to switch from solid to moored. Part of the benefit of the tower is that it can essentially be a big 'here I am' lighthouse. Could I still get up to a useful height for this with the floating platform? $\endgroup$ – jb6330 Apr 2 at 2:39
  • $\begingroup$ @MortenSickel Any idea how far out before we wind up with just sediments? Are my merfolk going to need to import stone for depths shallower than 350m? $\endgroup$ – jb6330 Apr 2 at 2:42
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    $\begingroup$ I would say if you cannot see land, there are probably no stones at the bottom... $\endgroup$ – MortenSickel Apr 3 at 13:06
  • $\begingroup$ Thank you for the hard numbers. Now I have something to work with/around. $\endgroup$ – jb6330 Apr 7 at 5:41
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Use Coral for the Spires

You could have a giant or fast growing coral that you either use as building material (coral can be as hard as concrete) or use it as the building itself, that could look cool as well.

A joint Building Project

The mer could also get help from the humans to build the top of the sea-spires with the mer building the bottom. They could also send building materials that the mer could not get themselves. You could have symbiotic relationship with the humans, they get a trade partner and a safe haven at sea, the mer get cool buildings that will bring in much needed industry and trade (like smiths and forges) the trade will get the mer items that the mer could never make in the sea

i would like to point out the mer have to be careful where they build theses sea-spires as the saying goes location location location. don't build it where the sea is highly active like storms, king waves, and earthquakes. be smart where you build them.

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  • $\begingroup$ Coral would definitely be a cool building material. Do you have any idea how deep it can grow or what kind of height I could support with it? Judging from what else I'm hearing, the mer would very likely be reliant on trade for other building materials if the coral doesn't work out. $\endgroup$ – jb6330 Apr 2 at 2:46
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    $\begingroup$ @jb6330 You can have the Coral as deep as the sunlight can go, other than that you will need to build supports for it to grow on. The height is a bit more difficult but just teat it the same as concrete and make as tall as you like. There are a number of ways to use the Coral either transplant or mold it like clay to the larger mass, or grow it from just one and shape it to what you want (use magic if you have to). Either way the Mer will be highly reliant on trade from the humans for basic items and tools. $\endgroup$ – Creed Arcon Apr 2 at 8:40
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Ancient Rome's Underwater Concrete

More than 2000 years ago, the Roman Empire invented a unique marine concrete that allowed for the construction of enormous, durable structures – even underwater. Incredibly, the exact chemical properties of this concrete mixture have eluded scientists to this day – but now, researchers from the University of Utah believe they may have finally cracked the code.

https://www.archdaily.com/875212/scientists-uncover-the-chemical-secret-behind-roman-self-healing-underwater-concrete/


The chemical secrets of a concrete Roman breakwater that has spent the last 2,000 years submerged in the Mediterranean Sea have been uncovered by an international team of researchers

https://newscenter.lbl.gov/2013/06/04/roman-concrete/


Here you see the Merfolk trading with humans from the top of their tower enter image description here

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    $\begingroup$ While this provides good background information, it does not appear to answer the critical question of "how deep can [towers in the ocean] be built". If the off-site links you provided contain that information, please summarize that information in the answer itself. $\endgroup$ – type_outcast Mar 31 at 13:00
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    $\begingroup$ That is a much better drawing than your anatomically correct strange woman in a pond! (SFW) $\endgroup$ – dotancohen Mar 31 at 14:45
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    $\begingroup$ @dotancohen - Well how good art is is a matter of opinion! However I have to admit that I found this illustration online. I'm not sure who the artist is. $\endgroup$ – chasly from UK Mar 31 at 14:55
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I want to frame-challenge the notion that merfolk could not manufacture materials that require being on dry land to make.

  • humans run refineries that are hundreds of feet tall, yet we do not have wings
  • humans run fish hatcheries, yet we do not have gills
  • humans run steel mills, yet we are not fire elementals

Just like all those things have catwalks or safety areas for the humans... a dry-land factory would have water-channels for factory workers.

So yes, they would simply dig up lime and set up a landside factory to kiln their own portland cement.

Of course if they trade with humans, all bets are off.

Another interesting wrinkle is nuclear power. Merpeople would struggle to build some of the machinery, but underwater is literally the perfect place for a nuclear reactor. It takes 99% of the "cooling problems that lead to meltdown" right off the table. You would have to labor to keep the reactor core, steam lines and turbine insulated (e.g. By keeping them in air spaces). At first sign of trouble, simply wet the insulation or flood the air spaces, rather easy if you are already underwater. Also, mining thorium/uranium is a fundamentally different task for seapeople; you don't search the world for veins, you simply extract it from seawater, and when you deplete your local sea, currents bring you more.

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