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Slide 21

After the 8 hours have passed, the driller then drills through the cement at the bottom and continues on down through other strata and the coal seam.  Again, for a short period of time the hole is not protected by a metal casing and cement.  If the surface casing and cementing is not right, then pollution can occur at this later time, though this is less common than the problems that occur before or during the setting and cementing of this first string of casing.