The Flight to Quality
That’s what traders call it when they become panicked and start buying up US Treasury bills, the safest investment you can get. Since so many people are buying them, the rate on them has fallen to zero, and maybe even below. Check out this line from a Goldman Sachs memo:
Yields on short-term Treasury bills dropped essentially to zero; in fact, some trades crossed at prices that implied investors were paying the Treasury to hold their cash.