I have the solution!
A wiki post formatted as such:
This is where we put ideas for solutions to the underwater civ predicament.
This is a wiki post so if you have an idea just edit it in.
Solution 1
A short summary of this solution.
For
Arguments for this solution.
Against
Arguments against this solution.
Solution 2
A short summary of this solution.
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Arguments for this solution.
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Arguments against this solution.
Solution 3
A short summary of this solution.
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Arguments against this solution.
… Alright I guess Hhyyrylainen put it here. I guess just put your solutions after the line below.
Hydrothermal vents
Use hydrothermal vents to smelt metal
For
The hydrothermal vents produce heat that could be used to smelt metal, maybe by building a forge around them to capture the heat.
Against
The vents get to hot for most large creatures to stand at all, it literally boils the water around it. For a creature to withstand that heat, they would have to have a thick carapace to block the heat, and even the they would not withstand the heat long enough to build something or care that some rock melted.