If built, the mammoth structure would be approximately five times as high as the World Trade Center was in New York.
Envisioned by Obayashi Gumi Corporation, Aeropolis 2001 is proposed to be a two-kilometer high building with 500 floors that would accommodate over 300,000 inhabitants. It's structural height would be 2,001 m (6,565 ft).
The structure is expected to be mixed-use, including restaurants, offices, flats, cinemas, schools, hospitals, and post offices. Elevators are expected to travel from top to bottom in 15 minutes. The proposed tower would be fully sustainable and air conditioned.
(Information comes courtesy of the Wikipedia amongst other sources.)