Cores obtained to the bottom of the Antarctic Ice Sheet at Byrd Station have been used to analyze some physical properties of the air bubbles trapped in the ice. These bubbles constitute the remnant air that is retained when polar snow transforms into glacial ice. Parameters measured included size, shape, abundance, and spatial distribution of bubbles, gas volumes, and bubble pressures and their variations with depth in the ice sheet. Bubbles occur abundantly in the top 800 m of ice but then gradually disappear until they can no longer be detected optically below 1100 m. This disappearance is not accompanied by any significant loss of air from the ice, and the available evidence suggests that the air is retained in the form of a gas hydrate or clathrate. Because of the release of confining pressures following drilling, the hydrate begins to decompose soon after cores are pulled to the surface. This decomposition is accompanied by the growth of gas-filled bubblelike cavities that are easily distinguishable from original air bubbles. Bubble pressure measurements show that (1) bubbles with pressures exceeding about 16 bars begin to relax back to this value soon after in situ pressures are relieved by drilling, (2) further slow decompression will occur with time, and (3) the rate of decompression is controlled to some extent by the intrinsic structural properties of the ice and its thermal and deformational history. Only small variations were observed in the entrapped air content of the ice cores; they probably reflect variations in the temperature and/or pressure of the air at the time of its entrapment. Only in ice from the bottom 4.83 m was the air content observed to decrease to trace amounts. Since this virtual absence of air coincided precisely with the first appearance of stratified moraine in the cores, it is concluded that this ice originated from the refreezing of air-depleted water produced under pressure melting conditions at the bottom of the ice sheet.