Welcome back, Wall Street. Investors returned to work to be greeted by tumbling oil prices and renewed worries about the world economy. U.S. crude hit a dubious new milestone, falling below $50 a barrel for the first time since spring 2009. The slick caused the Dow to slide 331 points yesterday and continued rattling global markets today. Adding to uncertainty is political turmoil in Greece, which could exit the eurozone. Expect more volatility as barrels of black gold dip into the $40s.