Investors clamored into the safety of U.S. government bonds, sending the 30-year Treasury bond yield below 2% for the first time ever and the 10-year Treasury note yield below 1.5%, a three-year low.

Around 2:00 p.m. ET, the yield on the benchmark 10-year Treasury note, which moves inversely to price, hit a three-year low of 1.475%, while the yield on the 30-year Treasury bond was at 1.944%, after earlier falling to 1.941% for the first time ever. The 2-year Treasury yield was 1.467%, its lowest level since Oct. 2017.

The historic drop in long-term U.S. bond yields comes shortly after interest rates on the closely watched 10-year and 2-year Treasurys inverted. The inversion of this key part of the yield curve has previously been a reliable indicator of economic recessions.

"The yield curve inverted which created a temporary 'pile on' effect in the bond markets," wrote Tom Essaye of The Sevens Report. "We have absolutely not seen what we wanted to out of the Fed. We had hoped for a rally in the 10-year yield and a widening of the 10s-2s spread. The exact opposite has occurred, and at this point currency and bond markets are no longer flashing 'caution' signs on the U.S.-global economy and risk assets, they are flashing a 'warning' sign —loudly."

That part of the curve was positively sloping again on Thursday but only slightly.

"The 30-year yield in itself is historic given that it is moving to massive lows but the curve inversion is typically the signal, one of the better signals you can get, that there is increased risk of recession," said Bank of America technical strategist Stephen Suttmeier.