A bottom assembly for drilling a deep bore from the surface into an earth formation with a pneumatic percussion hammer that advances a drill string including a plurality of connected reverse circulation drill rods, said bottom assembly comprising: a percussion bit; and a reverse circulation pneumatic hammer operatively connected to the bit; a reverse circulation stem rod having axially opposed top and bottom, and a radially outer surface, with the bottom rigidly connected to the pneumatic hammer; characterized in that the bottom assembly further comprises: a packer surrounding the stem rod, with a packer inner surface confronting the outer surface of the stem rod in an axially slideable relationship, and a packer outer surface radially spaced from the packer inner surface, said packer outer surface including at least one component which in a deactivated condition of the packer projects a first radial distance from the outer surface of said stem rod and in an activated condition of the packer projects a second radial distance from the outside surface of the stem rod, said second radial distance being greater than said first radial distance, and an activation mechanism arranged to activate said packer by applying force to said at least one component to cause said component to move from said first radial distance to said second radial distance, said activation mechanism being fixable in a first position where said force is applied until said packer is deactivated by moving said activation mechanism to a second position where said force is removed and said at least one component returns to said first radial distance, a radially projecting shoulder axially between the packer and the reverse circulation pneumatic hammer, wherein said axially sliding relationship between the inner surface and the stem rod outer surface is present when the packer is activated and deactivated and said packer is supported on the shoulder during advancement of the drill string to lower the packer to a position where the packer is activated. a percussion bit; and a reverse circulation pneumatic hammer operatively connected to the bit; a reverse circulation stem rod having axially opposed top and bottom, and a radially outer surface, with the bottom rigidly connected to the pneumatic hammer; characterized in that the bottom assembly further comprises: a packer surrounding the stem rod, with a packer inner surface confronting the outer surface of the stem rod in an axially slideable relationship, and a packer outer surface radially spaced from the packer inner surface, said packer outer surface including at least one component which in a deactivated condition of the packer projects a first radial distance from the outer surface of said stem rod and in an activated condition of the packer projects a second radial distance from the outside surface of the stem rod, said second radial distance being greater than said first radial distance, and an activation mechanism arranged to activate said packer by applying force to said at least one component to cause said component to move from said first radial distance to said second radial distance, said activation mechanism being fixable in a first position where said force is applied until said packer is deactivated by moving said activation mechanism to a second position where said force is removed and said at least one component returns to said first radial distance, a radially projecting shoulder axially between the packer and the reverse circulation pneumatic hammer, wherein said axially sliding relationship between the inner surface and the stem rod outer surface is present when the packer is activated and deactivated and said packer is supported on the shoulder during advancement of the drill string to lower the packer to a position where the packer is activated. a packer surrounding the stem rod, with a packer inner surface confronting the outer surface of the stem rod in an axially slideable relationship, and a packer outer surface radially spaced from the packer inner surface, said packer outer surface including at least one component which in a deactivated condition of the packer projects a first radial distance from the outer surface of said stem rod and in an activated condition of the packer projects a second radial distance from the outside surface of the stem rod, said second radial distance being greater than said first radial distance, and an activation mechanism arranged to activate said packer by applying force to said at least one component to cause said component to move from said first radial distance to said second radial distance, said activation mechanism being fixable in a first position where said force is applied until said packer is deactivated by moving said activation mechanism to a second position where said force is removed and said at least one component returns to said first radial distance, a radially projecting shoulder axially between the packer and the reverse circulation pneumatic hammer, wherein said axially sliding relationship between the inner surface and the stem rod outer surface is present when the packer is activated and deactivated and said packer is supported on the shoulder during advancement of the drill string to lower the packer to a position where the packer is activated.