Probably a few hundred years, so yeah, it wouldn't be worth it to the people doing it at all within their lifetime. That's why I was suspecting a far-sighted government or such with enough money to squirrel it away for future use.

The US sorta has a few things like that already--the oil reserve & the stockpile of helium, though I understand the latter is winding down still. Given the importance of those materials to science, I would think that there might be some scientifically-motivated project to protect our access to such things.

This is interesting. Can you point me to anything that will expand on/help me understand this?

I don't know of any good sources. But my understanding is that there are very few "Methuselah bonds" by which it is meant instruments that have lifespans of 50 years or more. So anybody with a profit motive has no advantage for putting money into something that would have a longer term focus. This explains to me why, for example, banks are still loaning money for buildings on Miami Beach. I would love to understand the dynamics better. It seems to me that only governments can be planning for longer terms. China, maybe?

Dykes are cheap.

I'm assuming you mean "dike" and not "dyke".

Dikes will not protect property in Florida. Our buildings are sitting on top of porous limestone.

oof yes. Dyke is not what i meant.

But florida already has a dike system. Hundreds of miles of them and they work quite well.