The development of large cities presents unique social, political, and technical challenges. As a city grows in population, the limited area must accommodate people and infrastructure. Additionally, economic development attracts even more people to large cities. With limited surface land area, these cities must create space in the third dimension: upward and downward. Larger cities are already full of skyscrapers, and the underground is typically used for utility infrastructure, subways, road tunnels, car parks, warehouses, and sometimes for activities of human life such as shopping centers, meeting places, and art galleries. Some cities are exploring the use of the underground space for offices, schools, and even residential buildings.