"The story truly is energy right now," said JJ Kinahan, chief market strategist at TD Ameritrade.
Crude dropped for a third straight day and touched its lowest price since August on expectations that supplies of oil will far outweigh demand. Even a report showing that the amount of supplies in U.S. inventories shrank last week did little to alter the tide.
Benchmark U.S. crude lost 98 cents, or 2.3 percent, to settle at $42.53 per barrel. Brent crude, the international standard, fell $1.20, or 2.6 percent, to $44.82 a barrel.
The price of oil has now dropped more than 20 percent this year, breaking into what traders call a bear market. How much of an impact that will have on most 401(k) accounts will depend on how much it undercuts energy companies' profits, and whether the pain will spill into other areas of the market.
"We're in the warning area here, between $40 and $44," Kinahan said of the price of oil. "If we get below $40, I think you'll get people adjusting their expectations."