The recession alarm bell ringing in U.S. government bond markets sent investors rushing once more to haven assets, pushing the world’s stockpile of negative-yielding bonds to another record.

The market value of the Bloomberg Barclays Global Negative Yielding Debt Index closed at $16 trillion Wednesday after the key U.S. 2-year and 10-year yield curve inverted for the first time 2007 -- a move often considered a harbinger of an economic downturn.

Global bonds surged on the bearish signal as investors sought the safety of government debt. The benchmark 10-year Treasury yield dropped below 1.6% and the 30-year equivalent fell to its lowest on record.

Strategists are increasingly speculating Treasury yields could join the below-zero club, something former Federal Reserve Chairman Alan Greenspan said wouldn’t be that big of a deal.