The original sense of ground zero is the point on the earth’s surface at or directly below a nuclear detonation. The term dates to 1946. From the Chicago Daily Tribune of 30 June of that year:
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Within a radius of one kilometer (.62 of a mile) from ground zero (the point beneath the blast center), men and animals died almost instantaneously from the tremendous blast pressue and heat; houses and other structures were smashed, crushed, and scattered; and fires broke out.
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