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The Earth Kingdom having earthbending-powered trains, the water tribe's Venice-like city and the wooden ships that are only outstripped by metal boats. Even firebenders developing more metal-based technology than anyone else makes sense since metalbending doesn't exist yet and advanced metalworking requires intense temperatures handled with precise control

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We see bending-powered tanks and ships well before we see metal jetskiis in season 3. What's more, the fire nation uses a combination of coal and bending powered technology, and they've been confirmed to have been using metal ships for a long time, which would imply that coal-power is a more recent development built to support non-bending and/or free up manpower for combat troops

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Though evidently this changes by the time of Legend of Korra