The 20th of April is not a day that hedge fund oil trader Doug King will forget in a hurry.

King, the chair of RCMA Group and head of RCMA Capital’s Merchant Commodity Fund, had been building up big bets that crude oil would fall. And that day, they paid off. The price of futures contracts for the May delivery of West Texas Intermediate dropped below zero for the first time ever — and WTI ending up plunging to -$37.63 a barrel.