Significant mechanical stress is built up between the surrounding diapir and the mine construction. Eventually liquids separate from crystals in the diapir and rise to the surface. If the rising salt diapir breaches the surface, it can become a flowing salt glacier. If the diapir is narrow its height must be greater. At this depth, salt rises and intrudes into the overburden, forming a diapir. Two stem from a diapir base, and the third from a sheet base. Extrusive advances begin once the diapir reaches the ground's surface and the salt is exposed. When one substance moves up through another denser substance, it is called a diapir.