A hard swell is a can that has swollen rigid with internal gas pressure — the end bulges firmly outward and cannot be pressed back into shape by hand. It is the advanced stage of can swelling, beyond the flipper (one end bulging, still pressable) and the soft swell. The gas comes from microbial spoilage — bacteria surviving underprocessing or entering through a faulty seam — or, less dangerously, from chemical hydrogen swells where acid foods react with the can metal. Whatever the cause, a hard swell means the can’s contents are suspect and the container must never be opened, tasted, or sold: botulism is the nightmare risk. Inspectors condemn hard swells on sight; consumers should discard them untouched.