U.S. Treasury bonds (and bills and notes) are often thought of as risk-free investments.
The U.S. Treasury has always honoured its debts in dollars. BUT the purchasing power of those bonds can decline, sometimes dramatically.
This chart, from Lyn Alden, shows the historical yield of the 10-year Treasury note (blue line) along with the forward 10-year annualized inflation-adjusted return from the start of that year (orange bars):
Negative bond yields are the primary driving force of higher precious metals prices.