Rising high above Lucca, Italy stands the Torre Guinigi -- the Guinigi Tower. It was built in the Fourteenth Century by a wealthy family as a demonstration of power. Oak trees planted in the garden on the top provide a s...

The Burj Khalifa in Dubai may be the world's tallest building, but not for long. China and Saudi Arabia already have plans to build taller skyscrapers. How long can this go on? Is there a physical limit to how high you c...