An ACIB is an air cushion device being developed in Canada for icebreaking purposes. When mated with the bow of a tugboat, the ACIB currently operational can break ice sheets about 1 m thick. During ACIB operations, the water surface directly beneath the craft is sometimes depressed below the lower edge of the ice sheet, and an air cavity is formed under the sheet. This air cavity is known to have a significant effect on the icebreaking capability of the ACIB. In a study for Transport Canada, DeHavilland Aircraft of Canada used underwater explosion theory to model its propagation. This paper shows that the explosion theory is incorrect. A more realistic theory backed up with experimental data is presented herein. The new theory is based on a theory for gravity currents developed by Benjamin.