The oil crash deepened on Monday, sending prices to an 18-year low in a stark demonstration of how the coronavirus is crippling fuel demand and leaving consumers unable to take advantage at the pump.

U.S. crude-oil futures slumped 6.6% to $20.09 a barrel, ending the day at their lowest level since February 2002. The drop brings their slide for the year to 67%, or $41. Oil enters the final trading day of March on pace for its biggest percentage drops on record for any month or quarter, according to according to a Dow Jones...