Then there are the unknown unknowns, as former Defense Secretary Donald Rumsfeld might say.

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Johnson said it is impossible to guard against all "unknown unknowns, " and that's why strong security is needed at venues.

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As Donald Rumsfeld might have put it, they have left a world of known unknowns for a twilight landscape of unknown unknowns.

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My job involved writing a lot of SQL queries, digging into unknown unknowns, and then formulating interesting hypothesis and stories to help steer executives in the right direction.

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What should really worry policymakers is the unknown unknowns.

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Executives believe there are no unknown unknowns.

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And so on into the unknown unknowns.

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Most tourists go to see the Tomb of the Unknowns, where an honour guard stands day and night over the remains of four unknown soldiers of later days: one each from the two world wars, the Korean war and the Vietnam war.

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