Sometime around early spring, many roads develop deep divots and pockmarks called potholes. Potholes form because asphalt road surfaces eventually crack under the heat of the day (after the rain) and the constant stresses of traffic.
In snowfall areas or further rain, these allow snow and rainwater to seep into the underlying dirt and gravel. During cold winter nights, this water freezes and expands. Some of the dirt and gravel is pushed out as a result, leaving a hole when the water eventually melts. Drivers continue to drive over these unseen holes, putting even more stress on the thin asphalt layer covering them.