[sa1]

It won't ever be economical to extract water from 600km below the surface. Also, oil won't last forever.

[thaumasiotes]

It's economical to desalinize seawater (once memorably [to me] described as "the most worthless substance in the world") now. We don't do it because using already-fresh water is even cheaper, but desalinization isn't exactly expensive.

[joshdance]

Source on the economics of desalinization? Everything I have ever seen have put it at pretty expensive. Would love to see a per gallon or liter price.

[dsaravel]

Well, the stone age didn't end for lack of stones.

[TeMPOraL]

Stone is a construction material, not a power source.