UAE, United Arab Emirates

The world’s tallest building stands on sand and is held up by friction.

Many believe that a tower like the Burj Khalifa in Dubai must be anchored in solid bedrock. But beneath Dubai there is no such rock at any reasonable depth, only sand and weakly bound stone.

So nearly two hundred concrete piles were drilled about fifty metres down. It works like a spade pushed into the sand on a beach, which stays upright: the sand presses on it from all sides and holds it fast.

It is that friction, between the piles and the sand and stone around them, that holds the tower in place.