Subsidence usually refers to downward motion of a ground surface, "settlement", but may also refer to below ground motion or internal subsidence. Caving is an often observed phenomenon associated with underground mining and begins with collapse of the back, roof, or hanging wall. Caving may occur unexpectedly or be deliberately induced in conjunction with the mining method. The transfer of load from a chimney cave to the walls of the cave zone leads to an elevated state of stress that increases the need for support in nearby access drifts, crosscuts, and raises. Trough subsidence is closely linked to excavation in stratified ground where extraction is 100%. Damage to surface structures from mine subsidence depends on the structure type and the motion transmitted to the structure. More accurate subsidence trough calculations may be obtained with the aid of computer programs designed for a particular coal field and calibrated against surface surveys over mines in the field.