America's shale drillers have never faced an oil bust quite like this. The split between Russia and its one-time OPEC allies last week has ignited an all-out price war. U.S. crude prices plunged the most since the 1990s, falling to less than $28 a barrel and rendering vast swathes of the U.S. oil industry unprofitable. Shares and bonds of shale producers plunged Monday, with the S&P 500 Energy Index posting its worst intraday decline on record.