Next time you go the beach, try a small experiment. Choose a flattish patch of damp sand below the high-tide mark but above the level of the sea. Then tap gently on the surface of the sand with the sole of your foot.
In an instant, the modest vibration of your tapping will be enough to turn what appeared to be firm, solid sand into a thick gloopy liquid that your foot will easily sink into.
What you will have just witnessed is a phenomenon known as liquefaction, in which a shock causes an apparently solid substrate to lose all its shear strength and turn into a liquid.
Liquefaction doesn't just happen on the beach. It also takes place on a much larger scale, for example when the vibrations from an earthquake hit land reclaimed from the sea.
If the shockwaves from a sizable earthquake were ever to hit Hong Kong, much of the land reclaimed from the sea in recent decades would turn into a similar gloop. That means the ground under Exchange Square and the IFC, as well as the Admiralty site on which the Hong Kong government's new headquarters is built, would instantly dissolve into the harbour.
No great loss, you might think. But many of Hong Kong's residential towers are also built on reclaimed land, including much of Tseung Kwan O, that would be similarly vulnerable.