Abstract: A new casing opening formation apparatus has been invented which includes an explosive charge and an apparatus for positioning the explosive charge in a casing in a cased wellbore at a desired location. In one aspect the apparatus includes a drill bit or other drilling device releasably connected to a bit diversion device; and in another aspect the apparatus includes milling apparatus for milling the casing. In one aspect the opening created is a radial ledge for facilitating casing penetration by a mill which mills out a window. In one aspect the opening created has such a radial ledge and includes a slot for inhibiting or preventing coring of the mill. In one aspect the opening created is such a slot alone. In one aspect the opening created is a completed window for sidetracking operations, no further milling is needed and drilling commences in the formation adjacent the explosively created opening.