By Stefanie R. Fishman, Project Assistant Potholes form when water becomes trapped beneath the pavement surface.
Water can enter the road base through surface cracks or from road sides.
The water freezes, often causing frost heaves. The ice melts from the top down, leaving a trapped pool of water. As vehicles run over it, the unsupported surface layer collapses. The pothole expands as traffic hits the hole. In the summer, highway departments can take preventive measures such as sealing cracks and improving drainage.