It is argued that deformation of the ice canopy has been neglected in past computations and considerations of the mass budget of pack ice in the Arctic Ocean. Evidence is presented to indicate that in the Canadian Basin alone, 13 to 18 percent of the ice area is covered by pressure ice considerably thicker than the 6 to 14 feet usually considered the average thickness of polar ice. A geometrical model based on empirical observations of under-ice, surface, and subsurface features is offered to permit computations of the volume of sea ice contributed by pressure ice. This model suggests that 1:3.3 is a realistic ratio between the above-water and underwater portions of an ice pressure ridge. Seasonal and spatial data from two years of aerial surveys on the frequency and height of ridges are presented, showing maximal values in winter near the North American coastlines. Similar data are presented on ice concentrations, stage of ice development, and the distribution of open and refrozen water throughout the Arctic Basin. Extensive submarine echo sounding data on the frequency of ice of various thicknesses corroborate the existence of larger amounts of open water - 15 percent more in summer - than previously estimated over large portions of the Arctic Basin. (Author).