Unity City
Unity City is where most things happen (think New York in Marvel). Located near the geographic center of the US, where Wyoming, Montana, and South Dakota meet. The city is densely packed and has many skyscrapers, the tallest of which being The Bell Tower, a massive office building with a functional replica of Liberty Bell at the top which is rung once every hour.
Unity City was founded in 1976 as a neutral center of the country and it is its own State. In a controversial decision, Unity City was given the right to expand it's borders as the city expanded outwards. After much protest by the surrounding states, some limitations were given to the country. This led to much of the city building upwards in order to save space. However, when the city is deemed to crowded, it is allowed to expand it's borders (and the states are compensated).
The total area of Unity State is roughly 25,000 sq mi (64,750 sq km). The population is 54 million for a population density of roughly 2,160 people/sq mi.