The time: 1955. The place: a dry lakebed in southern Nevada called Frenchman Flat. An explosion equivalent to 22,000 tons of TNT creates a roiling mass of superheated, low-density gas. This fireball rises and collides with the surrounding air, creating turbulent vortices that suck smoke and debris up from the ground into a column. The “stem” rises into cooler, thinner air, where the ascent slows, debris disperses, and moisture condenses to form a “cap.” Over days and even months, nuclear fallout spreads and drifts to Earth.