On 8 September 1941, Nazi forces surrounded Leningrad from the west and south, and through Finland to the north. A thin strip of land across Lake Ladoga kept the residents in touch with the rest of Russia, but heavy shelling made it impossible to evacuate the population. This was the beginning of the Siege of Leningrad, the costliest in terms of lives lost in history.

As the population starved, it was not unheard of for people to murder for ration cards and eat corpses. While the number of cannibalism cases were very few in proportion to the size of the population, the widespread fear of cannibals led to hysteria. The police even threatened unruly citizens with imprisonment in cannibals’ cells to maintain order. In many cases, corpse-eaters were often the most desperate people in society and largely involved single, unemployed mothers. Most of those caught for cannibalism were pitied and imprisoned rather than shot.

Despite people’s desperate condition, there was one source of food that remained untouched if you knew where to look. The Institute of Plant Industry’s gene bank in Leningrad was and remains the largest collection of seeds in the world: the most comprehensive catalogue of plants’ genetic information we have. Should any of the archived plant species become extinct in the outside world, these seeds, grains and tubers could be used to reintroduce them.

Even in their emaciated condition, the botanists at the institute defended the stores with their lives. They feared desperate people would storm the gene bank and eat their way through their life’s work, or invading forces would destroy the building to prevent its contents being used.

When the Red Army of the Soviet Union finally managed to lift the siege on 18 January 1943, almost two and a half years after it began, the seed bank was still intact.

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Should humanity face a similar crisis, whether a nuclear apocalypse or worldwide war, which foods might be safe for the survivors to eat, and how long will those foods last? To understand that, we need to ask what makes food go off.