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.