Bond guru Bill Gross believes the growing global move toward negative yields will have dire consequences.

In a tweet from his firm, Janus Capital, Gross goes back half a millennium to assert that the current situation with the world’s debt market is unprecedented and dangerous:

Gross: Global yields lowest in 500 years of recorded history. $10 trillion of neg. rate bonds. This is a supernova that will explode one day

— Janus Capital (@JanusCapital) June 9, 2016

The warning comes as yields on Japanese government bonds and German bunds hit record lows.

While it’s unclear what database Gross used to track bond yields back to the 16th century, there has been some academic research done on the topic. Bryan Taylor, chief economist for Global Financial Data, has done work on the subject and found generally that yields have declined over time.

In recent days, private banks have revolted over the growth of negative yields in Europe and Asia, a trend that has helped push big money inflows to U.S. corporate and government debt.

Gross runs the $1.4 billion Janus Global Unconstrained Bond Fund, which has returned 3.3 percent year to date.

Bill Gross negative yield bonds