We observed that the extent of bubbling of Ag-sheathed Bi₂Sr₂CaCu₂O_x tape varied with the partial pressure of oxygen (P_O2) in the surrounding atmosphere during melt processing. Tape that did not bubble when processed in air or 50% O₂ was severely bubbled when processed in 100% O₂ (P_total=0.1 MPa). When a tape was held at the maximum melt processing temperature ( approximately 890 degrees C) and the atmosphere was changed from air or 50% O₂ to 100% O₂, bubbles formed in the previously unbubbled tape. These bubbles often grew to be tens of centimetres in length and are called tunnels. The formation of tunnels is thought to be driven by the P_Co2 in the sheath from residual carbon in the powder. Increasing p_O2 is thought to either increase P_CO2 change the physical properties of the Ag allowing the sheath to deform in 100% O₂.