I imagine most of us have seen images or read reports about so-called "sinkholes." They occur when persistent rain washes away the ground beneath a road, creating cavities that are only precariously covered by the thin layer of the road surface. As soon as this surface is subjected to excessive stress—for example, by vehicles of any kind driving over it—craters open up in the road, the size of which depends on the amount of material that has been washed away.