In 1969 Erich von Däniken cited Delhi's rustless iron pillar as evidence of alien visitors.

402 · 1969 · apart: 1567 years

The answer is earthly: the ancient iron is rich in phosphorus, which grew a thin protective film of iron phosphate.

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The event

• Iron pillar of Delhic. 402 · Delhi. This seven-metre pillar weighing over six tonnes was forged under the Gupta king Chandragupta II. It has barely rusted: its high-phosphorus iron grew a protective film that in 1,600 years has thickened by just one-twentieth of a millimetre.

5th century · 20th century

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