The exploitation of coal or mineral deposits by underground mining methods. "Deep" is often interpreted as meaning 5,000 ft (1.5 km) or more, where stresses are high enough to cause sloughing of development openings, not to mention walls and faces in stopes. However, tectonic horizontal stresses are greater than gravitational force in many areas. Hence deep conditions can exist at lesser depths. Indeed, severe rock bursting caused the closure of a Canadian mine operating at depths of 500 to 700 ft (152 to 213 m). Also, rocks with low strength will produce deep failure patterns at modest depths, e.g., in Saskatchewan potash mines.