Crude oil prices below zero - why market prices were negative ?

Why there were negative market prices for West Texas Intermediate Crude oil on 20th of April and 21st of April ?

On 20th April the WTI crude oil prices dropped below zero for the first time in oil-market history. The reason of negative crude oil prices was high oil production and low demand for oil because of coronavirus crisis. Easily said, there was a global oversupply of oil. Crude oil storage space was filling up and it was also expensive to store the oil.

Major oil producing countries later agreed a deal, that they will reduce daily oil production to stabilize the crude oil market.