Only now is the real story of what happened that morning beginning to emerge. Russian scientists have revealed that tens of thousands of Altai people suffered sickness, cataracts, cancers and premature deaths because of one of the world’s worst nuclear disasters – and one of its best-kept secrets. The unfolding tragedy is already yielding new insights into the dangers of radioactivity.

The light in the Altai sky was a sign that, more than 170 kilometres away, the Soviet Union had surprised the world by exploding its first atomic bomb. Ruled only by Stalin’s urgent need to prove that he could do what the Americans had done in Hiroshima and Nagasaki four years earlier, Soviet bomb technicians had designed a crude nuclear test that was almost bound to be an environmental catastrophe.

They hung the bomb 33 metres above the ground on a gallows-shaped rig at the Polygon test site, just to the west of the city of Semipalatinsk (see Map), which is now part of Kazakhstan. The rig was nicknamed “the goose” because of its resemblance to the bird’s neck and beak. Crucially, nobody worried about the weather forecast or the wind direction. Apart from a few communities in the immediate vicinity, which were evacuated, nobody seemed to care about how the rest of the region’s population might be affected.