WASHINGTON — To the long list of economic mysteries can now be added interest rates. They’ve been at rock bottom, as everyone knows. But now we’ve encountered something novel: negative interest rates. Lenders are actually paying for the privilege of allowing someone to borrow their money. It’s occurring outside the United States, and the Federal Reserve’s next move is expected to be raising rates. Still, there’s no ironclad reason it couldn’t happen here.

Low rates are old hat. Here’s what Bloomberg showed on a recent day. Deposit rates for U.S. savers averaged 0.72 percent for one-year certificates of deposit and 1.5 percent for five-year CDs. On a 10-year U.S. Treasury bond, the yield was 2 percent. Abroad, some rates were lower. German 10-year bonds were 0.37 percent. British and Spanish 10-year bonds were about 1.6 percent.

Meanwhile, borrowers benefit. Rates on five-year auto loans were 3 percent; on 30-year fixed rate home mortgages, rates were 3.9 percent. But negative rates? How can that be?

In practice, here’s what happens. Bonds are traded on markets, just like stocks. Their prices can rise or fall depending on economic conditions or political events. When the price of a bond rises, its interest rate falls. Consider a $1,000 bond that was initially issued with a 3 percent interest rate. If the bond’s market prices subsequently rises to $1,500, the bond’s effective interest rate drops to 2 percent.

This is how bond interest rates can turn negative. If a bond’s price rises high enough, its original interest payments won’t cover the bond’s full market cost. “I buy a bond for $1,000 and get back $950 — that’s a negative interest rate,” says Moody’s Analytics economist Mark Zandi. In January, as much as $3.6 trillion worth of government bonds — mostly European and Japanese — had developed negative interest rates, estimate London-based analysts for JPMorgan.

Broadly speaking, there were two explanations for this, though they are not mutually exclusive.

The first is that negative interest rates, though unexpected, result from the easy-money policies of government central banks. Their bond-buying (known as “quantitative easing,” or QE) has poured money into financial markets, driving down rates. Although the Federal Reserve has halted new bond-buying, the Bank of Japan and the European Central Bank (ECB) continue their programs. The ECB has pledged to buy $1.3 trillion of bonds by September 2016.

Earlier in 2012, the ECB promised to stand behind the bonds of eurozone countries; this helped bring down their rates sharply. (Greece remains an exception, because its new government declines to endorse a rescue plan accepted by the previous government.) But hardly anyone anticipated that these measures would produce negative interest rates.

The second explanation is that the weak world economy has quashed inflation and the demand for credit. Businesses don’t want to expand; consumers fear too much debt. Weak global demand could produce a broad-based fall in prices (“deflation”), oil being a harbinger. Depending on deflation’s severity, negative interest rates could then be profitable because investors would be repaid in more valuable money.

The evidence for this theory is mixed. True, the sluggish world economy has suppressed price pressures. In the eurozone, consumer prices (minus energy) are up a mere 0.4 percent in the past year. But credit demand, while not robust, hasn’t collapsed. A study by the McKinsey Global Institute finds that worldwide credit grew 40 percent from the end of 2007 to mid-2014.

Just because bonds are traded at negative interest rates doesn’t mean there’s much buying at those rates. “I don’t understand why anyone would put up with negative interest rates,” says Richard Sylla, a financial historian at New York University and co-author of “A History of Interest Rates.” “You could do better by holding cash.” Some European banks now charge for holding cash deposits; in those cases, buying negative-interest bonds instead might make sense, says Sylla.

Capital flight by wealthy investors explains some demand for government bonds, says Zandi. “Every time there’s a hotspot you can see the cash flows into U.S. Treasury bonds — the safest assets,” he asserts. U.S. Treasuries aren’t yet in negative territory, but some other government bonds that play the same role are, Zandi says. Investors want to protect their principal and may regard slightly negative rates as an insurance premium against larger losses.

For the moment, negative interest rates are a market-driven curiosity. But what happens if governments or corporations begin selling bonds that start with negative rates? Then we’re in completely unchartered waters.

Robert Samuelson is a columnist for The Washington Post.