Why do potholes really form?
(Lansing State Journal, March 25, 1998)

Potholes occur due to the unique properties of water.  Usually when a liquid freezes, the molecules of the liquid pack very close together to become a solid.  This close packing is similar to what would happen if you filled a box with a bunch of balls of the same size.  The balls would very efficiently fill the box so that very little "empty space" would remain.

On the other hand, water behaves differently when it freezes.  Due to the bonding between water molecules, the molecules do not pack together efficiently.  Instead, the molecules in the ice have a cage-like structure that leaves more "empty space" than is found in most solids.
 
Potholes occur when water gets under the pavement.  When the water freezes the ice takes up more space than the water and the crack is enlarged.  The same phenomenon is experienced when you leave a can of pop in the freezer.  Since the pop is mostly water, the pop expands as it freezes and bursts the can.  When the ice under the road melts, a void is left, and the pavement falls in, forming a pothole.

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