When Metropolis was completed the Sanctuary City Alliance seems to have disappeared, whether they disbanded, moved into the Underdark, or onto another plane it is still a mystery. 

What is known is that all peoples started thriving, sure the lands outside the cities is still wildly dangerous and industries like mining, logging, fishing, and travel are far riskier than they would be on a calmer plane, but the industries advance nonetheless. 

The care and design of the cities fostered extremely fast adoption of the technomancy that was brought over from Eberron. A few hundred years later and we have airships powered by elementals who sign a contract to visit the material plane for a time and get shore leave when the ship is docked just like any other worker. Railcars powered by lighting, the Storm Transit lines employ storm giants while Blue and Bronze is run by a pair of dragons one blue the other bronze who both have a fascination with the short lived races that many dragons do not posses. 

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