Oil just did something that made even market veterans shake their heads in wonder — the soon-to-expire May contract for West Texas Intermediate crude on the New York Mercantile Exchange traded, and closed, in negative territory.

“I’m not sure how to react to that other than say that nobody, whether they’re 120 years old or whether they’re 20 months old, has ever seen an oil price lower than this,” Tom Kloza, a 40-year market veteran and head of global market analysis for Oil Price Information Service, told MarketWatch just minutes before the market closed on Monday.