The governor of California has declared a state of emergency in a suburb of Los Angeles over the leaking of methane gas from an underground storage field.
More than 2,000 families have been moved from their homes and many people have reported feeling ill because of the leakage, which began in October.
It stems from a vast underground storage field in Porter Ranch, on the outskirts of Los Angeles.
Gas is spewing into the atmosphere at a rate so fast that the well now accounts for about a quarter of the state's total emissions of methane - an extremely potent greenhouse gas.
The well is situated in a mountainous area more than a mile away from residential areas, but residents have complained of health effects like headaches, nausea, vomiting and trouble breathing.
"Let's call it an environmental and public health catastrophe," Tim O'Connor, a lawyer with the Environmental Defense Fund, told the BBC's environment correspondent, Matt McGrath.
"In terms of timelines this is going to surpass the gulf oil problem by a mile. What we do know is that that climate equivalent of this leak is like burning thus far almost 700 million gallons of gasoline or it's the same amount of pollution as 4.5 million cars put out every day, it's tragic."
Note EU-Digest: It is scandalous that this problem which has been going on since October is only now being publicized in the US
Read more: California state of emergency over methane leak - BBC News