Copper has plunged to a 5 1/2-year low, in the latest example of slowing global growth hammering industrial commodities.
In the overnight session, copper fell 7 percent to the lowest level since July 2009 before recovering somewhat. Still, the red metal is down a stunning 9 percent this week.
When it comes to the speedy down move, in which copper futures fell 6.7 percent from 8:20 p.m. ET to 9 on Tuesday night, few ready explanations suggest themselves.
"There was no particular trigger behind the fall, but we should not underestimate the power of Chinese fund money to roil the markets," wrote metals analyst Edward Meir of INTL FCStone. He adds that bearish put options on copper have been active, and "presumably, some fund players may have sold more copper in order to tip those options into the money."
"All in all, [there's] quite a lot of firepower lined up on the short side," he surmised.
Still, no matter the cause for the short-term action, the long-term case against copper is clear.
"China is the biggest user of it, the biggest consumer. Manufacturing has slowed in China, according to the latest stats.… So the demand for copper has been way down for the world's biggest user," Anthony Grisanti of GRZ Energy said Wednesday on CNBC.
The problem stretches beyond China, however. The World Bank cut its global growth forecast to 3 percent from 3.4 percent in a Tuesday report, which is bad news for copper demand. Copper is often thought of as a global growth barometer, as it is used in everything from homes to cars to electronics to factories.
For that reason, Scott Nations of NationsShares says that copper's plunge is actually more concerning than oil's.
Crude's drop is driven mostly by supply, but copper's drop "is being driven by demand," Nations said. And "if growth is running on fumes, then our stock market's in trouble."
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