Water can seep under pavement through cracks or from the side of the road. Over time, the water can cause the material under the pavement to erode, causing the pavement to sink down and break. During the winter, the water under the pavement freezes then thaws (contracting and expanding). This freeze / thaw cycle can cause the pavement to crack. Once cracked, the pavement deteriorates quickly under the weight of traffic and streets can seem to break out in potholes overnight.