Where: Jeddah, Saudi Arabia

Estimated completion: 2012

Conceived by Prince al-Walid bin Talal

If completed the building will rise up more than a mile in height providing those (without vertigo) a view of the Middle East, North Africa and the Indian ocean all from one spot.

At 5,250ft, the £5 billion project will be twice as high as the competition. During its construction it is thought that the construction workers will be brought to work every day via helicopters!

That's just one of the many technical challenges the designers will have to overcome, but what are some of the world's greatest wonders that got past the drawing board?