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In Kim Stanley Robinson's excellent sci-fi trilogy, Red Mars, Blue Mars, and Green Mars, they build theirs out of a mix of nanotubes and diamond. Larry Niven also has them in his sci-fantasy Rainbow Mars. I think it's totally feasible eventually, although anchoring it to the seabed is a new one on me. Clarke even talked about floating ocean platforms instead of his original idea of land anchors, to move them out of the way of asteroids.