One of the key problems with making really tall buildings, is that you have to get all of the people in and out of them efficiently. Most of the world's highest skyscrapers use express elevators to get you up to the top floors quickly, but as your elevator shafts get longer and longer you run into a serious problem. To run an elevator that high you need really long elevator cables, and with traditional steel cables, that means really heavy cables. In fact, once you reach about 500-meters, or the height of Taipei 101, the cables by themselves weigh three times as much as the fully loaded elevator car plus its counterweight.
To solve this problem, the Finnish elevator manufacturer Kone has developed UltraRope, which replaces the old steel cables with a flat ribbon made from carbon fiber. Kone claims that it is twice as strong as steel and lasts longer, yet it is 90% lighter than the old cables. Kone says that this makes one kilometer tall buildings economically feasible, while some architects claim that even a mile high building would be possible. Now that really would be worthy of the name skyscraper.
Check out the video to see how Know developed and tested UltraRope.