Central bankers hinting at more monetary stimulus have depressed yields so much that even some European junk bonds are trading at levels where investors have to pay for the privilege of holding them.

The number of euro-denominated junk bonds trading with a negative yield—a status until recently associated only with ultra-safe sovereign borrowers—now stands at 14, according to data compiled by Bloomberg. At the start of the year there were none.

Cheap-money policies since the financial crisis have kept interest rates for the past decade at, or near, all-time lows. That's prompted many investors to buy riskier assets that yield enough for them to meet their liabilities, driving bond markets higher and yields lower. The European Central Bank (ECB) said on Monday it's ready to add more stimulus to the Eurozone, indicating that an end to the age of ultra-low borrowing costs is far from over.