The cost to have a barrel of U.S. crude delivered in May plummeted to negative $37.63. It was at roughly $60 at the start of the year. Traders are still paying $20.43 for a barrel of U.S. oil to be delivered in June, which analysts consider to be closer to the 'true' price of oil. Oil is traded on its future price; the May futures contract expires Tuesday. That exacerbated volatility; by Monday evening US oil prices were back above zero, reports said. Monday's sharp decline is a result of May's futures contract closing Tuesday, when trading contracts for it expire and the earliest delivery they'll be able to buy is for June.