Walter Gregg was a railroad conductor and a World War II veteran. He and Effie had built a quiet life with their three children, Walter Jr., Helen, and Frances, on a rural property near Mars Bluff in Florence County, South Carolina. Their niece Ella Davies, age nine, was visiting that day. Walter took the worst of the injuries - a deep cut to his right arm. Effie was struck by a dislocated piece of plaster and cut on the head. The Gregg children took flying debris and were left with mostly superficial wounds. Ella's injuries were more serious; she required surgery for internal bleeding. The family chickens were not so lucky. Several were killed outright by the blast. The Gregg house was a ruin. But the people were alive, and that is the part of this story that matters most.

The bomb came from B-47E aircraft 53-1876A out of Hunter Air Force Base near Savannah, Georgia, bound for Britain as part of Operation Snow Flurry. Captain Bruce Kulka was the navigator-bombardier. Somewhere over the Pee Dee region, a warning light indicated the bomb-locking pin was not engaged. Kulka climbed back into the bomb bay to inspect it; in the cramped space he grabbed for a handhold, accidentally took hold of the manual release lever, and the Mark 6 hydrogen bomb - a weapon weighing about 7,600 pounds and capable of a yield in the megatons - dropped through the bomb bay doors and out of the aircraft. Crucially, the bomb's fissile core was stored separately, as was U.S. practice during peacetime transit. What fell on Mars Bluff was a nuclear weapon without its nuclear charge. What detonated was the several thousand pounds of conventional high explosive used in the weapon's design - more than enough to flatten a house.

The blast blew the playhouse to splinters and lifted Helen and Frances and Ella into the air. It collapsed the Gregg home around Walter and Effie. The shock wave damaged buildings within a mile and was heard for many more. When the Air Force crews arrived, they found a crater seventy feet wide and thirty feet deep in the Gregg's garden - measurements that have been reported variously in different sources, but in any version, a substantial hole where the family's tomatoes and beans had been growing the day before. The B-47 circled back, the crew confirmed the loss, and the message went up the chain of command in language that has become an icon of Cold War understatement: aircraft 53-1876A had lost a device. Walter Gregg later sued the Air Force and received about 54,000 dollars - far less than he had asked for, far more than military families typically got.

The New York Times reported that this was the first time a nuclear weapon had been accidentally dropped on the United States outside testing grounds. It was not even the first that year. A few weeks earlier, a B-47 had collided with an F-86 over Tybee Island, Georgia, and jettisoned a hydrogen bomb into the Atlantic that has never been recovered. The Department of Defense's later "Broken Arrow" inventory of serious nuclear weapon accidents lists at least thirteen before Mars Bluff. The Mars Bluff incident did prompt one quiet operational change: nuclear weapons aboard U.S. aircraft were thereafter more securely fastened, making it harder to release them - accidentally or intentionally. The fix took an afternoon and a hydrogen bomb landing on a family's playhouse.

The crater filled in over the years. The Gregg family rebuilt and stayed in Florence County. Walter Gregg died in 2003 at age eighty-one; Effie died in 2010. The site is on private land and not generally accessible, but a roadside historical marker was placed nearby in 2008, and the Florence County Museum in downtown Florence has a full-size replica of the Mark 6 weapon on display, along with shards of the original bomb casing. Visitors stand and look at the replica and think about the family who survived this. The technical lesson was about bomb-locking pins. The human lesson is that Walter and Effie Gregg's children grew up, and grew old, in a country that had just barely - by the absence of one carefully stored core - not detonated a hydrogen bomb on top of them.