Richard Drew/AP

The 10-year Treasury bond yield dropped below 1% for the first time in history, as investors grew increasingly worried about the novel coronavirus outbreak.

US government bond yields have been trending lower since the start of the year, but their fall accelerated as US financial markets reacted strongly over the past couple weeks to the global outbreak.

Exacerbating those fears, the Federal Reserve delivered an unscheduled half-percentage point interest rate cut today to boost the economy in the face of coronavirus. It was its first emergency cut, and the first cut of that size, since 2008. Investors don't believe it will have been its last.

The US stock market tumbled today, with the Dow tanking more than 900 points, as investors balanced the economic jolt of the Fed's rate cut with the statement the cut makes about the potential damage coronavirus could do to the American economy.