A barrel of U.S. crude fell below $40 per barrel for the first time since the end of the global economic crisis.
Friday's fall, to $39.86, was just the latest indicator of a vast shift in the energy landscape.
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Oil prices have been falling solidly for eight consecutive weeks. That's the longest streak since 1986.
Prices have fallen almost 60 percent since this time last year, and more than 34 percent in just the past three months.
The U.S. is churning out oil at an unprecedented pace, adding to the supply from energy powerhouses like Saudi Arabia and other OPEC nations.
What may be pushing prices this low, and possibly lower, is a steady drumbeat of economic data out of China suggesting that the world's second largest economy is slowing.