Part of the answer is that a growing chunk of American and German bank notes circulate outside these countries, in such places as Russia where the local currency is not trusted.

ECONOMIST: Cash remains king

The yield on Portuguese 10-year notes jumped to a new crisis high of 6.33%, with the spread over German 10-year notes rising by 3.5 percentage points.

WSJ: Dow Takes a Harrowing 1,010.14-Point Trip

Yields on German 10-year notes fell to 1.175%, a record low, touching 1.123% intraday, while Sweden's 10-year bond yielded 1.166%, according to Tradeweb.

WSJ: Dow's Drop Is Worst This Year