(Bloomberg) -- Among the many scary things traders witnessed as stocks plunged last Monday, one of the most harrowing was the sight of the VIX, an index designed to measure investor fear, briefly going dark.

For almost 30 minutes as hundreds of billions of dollars were erased from equities, no signals were sent by the world’s most popular sentiment gauge as options prices turned erratic. When it switched on, the VIX jerked higher faster than anyone had ever seen, rising 82% on its first tick to 51, a level not reached since the financial crisis.

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