Roads, highways, airport runways, railways, and levees (dikes) all consist of carefully designed and built layers which ultimately rest on an earthen base. Over time, erosion, nearby construction, weather, seismic activity and other causes can weaken or create gaps in the subgrade. Initially there is no sign of this on the surface, until a collapse suddenly occurs. This collapse entails expenses for immediate repair, and direct and indirect costs to the surrounding communities due to disruption of transport and damage to buildings. In some cases there can be environmental damage or human casualties.