The proposed 220-story Sky City to be built in China would be taller than the Burj Khalifa but take only nine months to build, according to Broad Sustainable Building. However, critics worry about subsidence and safety. The skyscraper would have 4,000 apartments, a hotel, offices, school, hospital, stores and restaurants. "With so many people living and working in the building, there will be risks everywhere," said Li Xun of the China Academy of Urban Planning and Design. However, the developer said its "design team is professional and has taken all safety factors into consideration."