Scott Adams' "Barge World"

I love to think about different ways people could live in the future that are radically different from how things are now. Scott suggests a "barge world", in which a large portion of the population lives on barges (flat-bottomed boats, usually used for transporting goods, which can be connected together somewhat like a train to expand). People can float on their own or connect together to form temporary or permanent cities, taking advantage of the additional 70% of the Earth's surface which is currently uninhabited. There are a couple of disadvantages to this, of course; for instance, if a substantial portion of the ocean surface was covered with barges, it would block light for the ocean life underneath. But it is an interesting concept, and probably plausible with proper engineering and foresight.

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