This image appears in the 1984 book The Future World of Agriculture and illustrates futuristic farming techniques near a sea city.

Robots tend crops that grow on floating platforms around a sea city of the future. Water from the ocean would evaporate, rise to the base of the platforms (leaving the salt behind), and feed the crops.

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See also:
Sea City 2000 (1979)
Robot Farms (1982)
Farm of the Future (1984)
Superfarm of the Year 2020 (1979)