As a marine engineering fanatic I want to agree and have done a decade long deep dive into the topic (pardon the pun) in search of an economic rationale for underwater colonies. I came up mostly empty handed.

Shirtsleeves underwater habitat space is very very expensive. Not as expensive as space but still not cheap. There are only a few things you can do underwater at a profit that actually need a habitat, or which can be done more efficiently with one.

These include mariculture, science, tourism and mining. All of these can be done with a very small team of aquanauts, less than a dozen in every case, in some cases half that figure. While I would love to be wrong, economics rules, and economics says the future will have many small underwater stations for scientific and business purposes but no large underwater colonies/cities.

If extra living space is what you're after, and you specifically want or need to live at sea, seasteads are a much more economical approach. Because covid basically destroyed the cruising industry recently there's a lot of aftermarket cruise ships for sale. One has already been bought and renamed the Satoshi, to be converted into a permanent floating community where only crypto is accepted as payment

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