When water phase-transitions into ice it expands. The water usually contains dissolved air. Freezing forces the air out of the solution into bubbles.
Are these bubbles at a lower or higher pressure than atmospheric pressure?
I can see arguments both ways: when a washer expands, both the outer and inner radius increase. This would imply bubbles are at a lower pressure, but perhaps the ice expands differently and squeezes the bubble and it is at a higher pressure. However, if they were squeezed I would expect them to try to expand, and propagate along weak fraction lines in the ice, and I do not see this - bubbles are usually round, suggesting low pressure environments.
Edit: Two papers on the subject, but neither sheds direct light on the answer:
"Bubbles and bubble pressures in Antarctic glacier ice" (A. J. Gow, J. Glaciol. 7 no. 50 (1968), pp. 167-182) shows that the bubbles shrink with increasing pressure. These are therefore presumably high-pressure bubbles. However, the internal bubble pressure is due to ice compressing as more ice forms on top, not just the freezing process.
"Air Bubbles in Ice" (A. E. Carte, Proc. Phys. Soc. 77 no. 3 (1961), p. 757) talks about bubble formation, but not internal bubble pressure.