RWBY: WORLD OF REMNANT
See, before the Great War, there was no Atlas - there was Mantle. At some point in time, a group of settlers were crazy enough to venture out into the northernmost continent of Solitas. I guess when you're that desperate, a frozen hunk of rock doesn't seem like such a bad place to call home. As a matter of fact, the harsh weather conditions proved to be just as useful as the mountain ranges when it came to keeping the Creatures of Grimm at bay. It also kept the people of Mantle from flourishing.
Humanity's got a neat trick up its sleeve, though. Whenever we're faced with a problem, our inclination is to find a solution. The cold climate of Solitas forced its settlers to adapt. It developed a more advanced technology - and they did it faster than the rest of the world - because they had to to survive. But it was the Great War that really kicked things off.
New forms of Dust application and weaponry allowed Mantle to expand. More and more territory was set aside for Dust mining and research. The territory beside the Kingdom's combat school, Alsius, was the most opportune area to construct a new R&D facility. But by the time things were all said and done, it would be much, much more.
Alsius was reopened as Atlas after the Great War, to house many of the warriors now seeking guidance. In an effort to "give back" to the citizens, Mantle applied all sorts of new Dust techniques and technologies it had used after the war to beef up the Academy's campus. School grounds expanded faster than they could have imagined, and even helped to better secure the surrounding areas. It wasn't long before the Kingdom's military moved in, then the labs, and research facilities. Eventually, even residential areas started popping up.
In time, it became apparent that the city of Mantle was living in Atlas' shadow. And so the decision was made to move the Kingdom's capital. Mantle was old news - and the Kingdom of Atlas was born. "A golden age of prosperity," they called it - but those left behind in Mantle would probably tell you it was the coldest winter they ever knew.