A scientist in blue jeans and cowboy boots leans over a sphere of plutonium capable of leveling a city, prying apart two reflector shells with the blade of an ordinary screwdriver. This is the true story of the Demon Core, a 14-pound Manhattan Project plutonium sphere originally destined to be the third atomic bomb dropped on Japan, that instead claimed two lives in peacetime laboratory accidents.

We trace the core's eerie alternate history, the physics of criticality that left only a razor-thin safety margin, and the fatal hubris of 'tickling the dragon's tail.' Through the deaths of Harry Daghlian and Louis Slotin, we see how genius and past success offer no defense against the indifferent laws of physics.

• Why the plutonium-gallium alloy and nickel coating were engineered to stay a stable, machinable sphere
• How a droppe ...