The massive city and life's work of the High Evolutionary. It was built underground so he could experiment without prying eyes, planning to raise the city once he had completed and perfected his work. He built an ascension engine to accelerate the evolution of all those who stepped into the city. But the engine was a failure, causing the opposite effects. The High Evolutionary and his New Men left the city but never shut the engine down. It then became known as the Abandoned City of the High Evolutionary.
But the engine needed to time to start working properly. When the Subterraneans stumbled on the city, it made them devolve in appearance closer to that of humans (since they are merely evolved humanoids). The Moloids took over the city as their own. They soon found that when they bred, their children would not be evolved and they were shunned and treated as animals (despite having evolved to hyper intelligence).
The Mole Man got the help of the Fantastic Four to stop them from raising the city, but they were unsuccessful. Rising to the surface in the United States, the city wishes to be declared it's own independent nation.