#omashu

omashu?.. you mean..

the city founded in honor of star crossed lovers, whose villages were enemies?..

who wore red and blue clothing?..

...

My best guess is that they use Earthbending to dig very deep wells into a massive aquifer under the city.

One possible evidence of the aquifer's existence is the Foggy Swamp, which is nearby-ish to Omashu and is also nestled in the mountains.

And yet it's got plenty of water, probably drawn to the surface by the roots of the banyan tree.