So, I saw the FAQ, and the part about underwater civilization.
What I realized was this:
Even under-water, a civilisation could still do metal working, just using geothermal vents. So, maybe a new property: heat. A civilization could use any source of heat. Typicly fire, yes, but geothermal, mirros (Sunlight), and so on
Just a thought!
This is my underwater civ idea
In order to efficiently use a hydro-thermal vent to smelt, an animal would have to basically be adjacent or inside the vent, continuously exposing itself to extremely high temperatures for a length of time no creature on Earth could survive.
It wouldn’t make sense for a complex multi-cellular organism to evolve this. Yes, many animals can withstand high temperatures. No, not to a hydro-thermal vent’s extent; it wouldn’t make sense for an animal to evolve a heat threshold up to the capacity of withstanding lava unless that animal depends on lava to survive, which would be too dangerous and erratic of an environment to develop that relationship in the first place.
I could maybe see some sort of crab-like animal inhabiting a shallow sea developing metallurgy, but solely based on their ability to go on land. So I think it should technically be possible to have an “underwater” civilisation, but only after a certain point of living on land at which an animal is advanced enough to make technology that can smelt underwater, like an advanced version of Greek Fire. Besides that, I wouldn’t think a purely underwater from start to finish civilisation would be possible.
This has been suggested in the past. So much so that it is a meme. I would have posted the video if @Deus hadn’t posted it already.
Also I moved this discussion to the relevant thread.
I think about amphibian civs, and … How about 2 civs (underwater and air), that will be in symbiosis 
air civ give metals, underwater give food, jewels (as pearls) or knowledge.
I like the idea but it’s highly unlikely the two civilizations could be competitive enough to leave their planet but also not competitive enough to kill each other
Yeah, the tech isn’t easy, but fire isn’t the only way to melt metal. A society that is advanced mentally, but ot technilogicly, could make glass and melt silver in a vent. They could them use that to make lenses and mirros, and use the star to melt.
I doubt it would be easy, so, it could be a “challenge mode”!
and, @Magic8Ball04 they wouldn’t have to be competitive. They could just go into space out of sheer curiosity, working together! (The water ones have water tanks, not air tanks. Like a walking aquarium 
)
That’s legitimately cute and I want to see it happen
what do you mean “cute”
Iron in the earths core is kept at a liquid state via pressure and radiation
planets near stars are warmed up via radiation, sometimes enough to form magma
iron in modern day forges (The factory ones) are kept molten by either carefully stocked fire, or electricity
in other words, it takes thermal energy, however it’s generated (star, uranium, burning), and needs to be transferred (contact (fire touches stone/ smeltery, touches metal), convection (Air currents), radiation (Electromagnetic (Star, Microwave), or particle)
So, in summary, what did you mean?
I mean that the symbiotic relationship that you defined was wholesome and I want it to happen.
I get what you’re getting at now! Maybe, eventually, a symbiotic relationship will form with humans (Granted, all the same species, but groups of people can be so different), but…
so, thanks!
Here are my notes that have sat for quite a few months:
Internal temperature = 35 C
Water Temperature = 650 C
Temperature regulated with vent like structures centralized on the lower rear of the body that cools them. They can be shut the access points at will or through instinct.
q (conduction) = -kA(T/x)
Temperature difference = 615 C = T
Silicon Shell = 1 inch thick = x
Cross sectional area = 3619.11 - 3179.29 = 329.82 in = A
Thermal conductivity(k) = W/(mK) watts per meter Kelvin
W=VA (joules per coulomb x coulombs per second)
Joules = 1234415.35
coulomb info undetermined
I really admire the effort you put into this, but there’s one thing I cannot understand. From my understanding, wouldn’t the shell have to be seamless for it to have a considerable effect? That seems unlikely, also, how would we prevent the heat to just start cumulating inside the shell? Since from my understanding, it is supposed to be a good izolant. Wouldn’t that cause the body heat to just accumulate inside the shell, boiling the creature like a greenhouse gas effect?
Again, I admire your effort and you have clearly done more reading on the topic than me, I’m just asking the kind of annoying questions that might help to bring the idea together.
Some things: it’s not W = V * A, it’s W=V * I, or W=I^2/R,
where
W= wattage (watt)
V= voltage (V)
I= current (A)
R= resistance (ohm) (not a very large issue, but still something I had to point out)
And might I ask where the value for joules comes from?
Last of all, W is not electric wattage, since it’s not about electricity but heat, so these formulas don’t even apply in the first place. Can’t you just use a constant from the internet for K? It’s just silicon right?
And then using that, you can calculate the time before the creature heats up too much, and then try to find out if that’s enough time to get some smithing done.
I put somewhere in there that there that there is a series of vent like structures that all connect and circulate through. I have two options and I need to google the properties of certain elements more but it will either circulate cool water which is kept cool by said organ or a (essentially) bio coolant made of organic compounds. These creatures spend about 2/3 their lives outside of the scorching hot ocean areas. They flee there from predators that would boil alive if it chased, they also lay their eggs on the outer edges (still too hot for other creatures but cool enough to protect the eggs). They aren’t the only creatures that have adopted this strategy of course but all of the species that live in the volcanic zones are scavengers and don’t fight eachother. I’ve actually developed the biosphere a bit as well as the geological processes of the planet.
I know one very good game, that names “The longest Journey” (first, in second I not played). Very good game. In this game was two very interesting rases - alatins and maerums. Alatins - flying creatures and maerums - creatures like mermaids. They are interesting because they are from one rase and from space.
I need a thermal conductivity equation and this was the one I found. That equation seems to be electric conductivity.
I know, but I have literally no clue where ‘W = V * A’ comes from, due to you not having put what they mean. Joules/coulomb and coulombs/second is what volt and ampère stand for, which have V and I as quantities. I feel like ‘volume * surface area’ is not what it’s is supposed to mean either.
In this case, I looked it up, and q should be Q/t, or W (or H), all of which is in joules/second (or watt).
The entire W=VA part might as well just be removed, since it both either uses different terms for quantities than the SI, or is just absurd (volume * surface area?), and I have no idea what its use is even supposed to be.
EDIT : It could be useful to build an underwater city if colonizing a water-only planet or an icy satellite with an underwater ocean.
Maybe not in our terrestrial style, but an underwater environment has very different design constraints. The high density of water allows many things to float which normally fall in air.
The “tarp cities” of the Extremely Bad Thrive Comics have basis in reality - underwater huts don’t need precisely-placed sticks to stand upright, merely some buoyant cloth, some rope, and something heavy to tie the rope to.
Perhaps we just aren’t used to building with 3 free axes instead of two, as gravity is much less impactful underwater.