At around 8.30 a.m. London time, the yield hit zero and briefly fell into negative territory as investors continued to flock to safe-haven assets. Bond prices and yields move in opposite directions and a negative yield implies that investors are effectively paying the German government for the privilege of parking their cash.

By the end of the European trading day, the yield was still just in negative territory at -0.0020 percent.

A spokesperson for the German Federal Debt Agency spoke immediately after the milestone was reached, stating that the tradability of federal securities is "still very high."

"The federal debt-management strategy is long-term, therefore, the current absolute yield level plays only a subordinate role. Our target remains a sustainable balance between cost and planning security for the debt portfolio," the agency said in an email to CNBC.