Less than a month later, Walter Gregg and his son, Walter Junior, were in the toolshed outside their home in Mars Bluff, South Carolina, when a Mark 6 atomic bomb landed in the yard. Mrs. Gregg was inside the house, sewing, and her daughters, Helen and Frances, aged six and nine, were playing outdoors with a nine-year-old cousin. The Mark 6 had a variable yield of anywhere from 8 to 160 kilotons, depending on the type of nuclear core that was used. The bomb that landed in the yard didn’t contain a core. But the high explosives went off when the weapon hit the ground, digging a crater about
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