The price of benchmark European natural gas futures has dropped 20% since last Thursday, and by more than 70% since hitting a record high in late August. On Monday, Dutch gas spot prices for delivery within an hour -- which reflect real time European market conditions -- dipped below €0, according to data from the Intercontinental Exchange.

Prices turned negative because of an "oversupplied grid," Tomas Marzec-Manser, head of gas analytics at the Independent Commodity Intelligence Services (ICIS), told CNN Business.