A city center built in an excavated cavern under the Appalachian mountains. Five miles wide, fifteen miles long, a quarter-mile tall. The roof of the cavern is supported by narrow towers made of super-strong composite material.
The floor of the cavern is covered with tall buildings and parkland, with a street layout reminiscent of Manhattan. The population of the city center is about 3 million.
The walls and ceiling of the cavern are covered with screens that show a perfect stereo image of the sky.
Surrounding this city center are smaller caverns containing suburbs, industry, and agriculture. All are connected by thousands of transportation tunnels that autonomous cars and trucks move through. The total population is around 20 million.
Over time, the city is working towards 100% sustainability, including fully recycling water and air. Power is piped in from a giant floating island in the Atlantic, covered in solar cells.