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London (CNN Business) Want to know just how serious a blow the coronavirus pandemic has dealt to markets and the economy? Look at the price of oil.

On Monday, US oil to be delivered in May settled at -$37.60 per barrel, the first negative close in history. That means producers, which are running out of storage space as demand for energy collapses, are willing to pay buyers to take crude off their hands. That's never happened before for West Texas Intermediate futures, the US benchmark.

The latest: The price of a barrel of WTI crude to be delivered in May is still below zero. The contract closes on Tuesday, which means trading is light and most investors are looking ahead to June. But the price of a barrel of WTI crude to be delivered that month is also plunging. It's down 18% on Tuesday and is now trading below $17 per barrel. Brent crude futures, the global benchmark, dived below $20 per barrel to their lowest level since 2002.

"[I'm] shell shocked," Bjornar Tonhaugen, head of oil markets at Rystad Energy, told me. There is normally some volatility at the end of a contract period. But this indicates that the oil market is "officially broken" as storage piles up, he said. There's particular concern about how much space is left in Cushing, Oklahoma, the delivery hub for WTI.

The effects of dirt cheap crude will ripple through the economy. Even in a $20 oil environment, 533 US oil exploration and production companies will file for bankruptcy by the end of 2021, according to Rystad Energy.

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