What sent a chemical storage tank in Garden Grove, California, to the verge of either leaking or blowing up?

Orange County fire officials said the threat of a catastrophic vapor explosion occurring at a chemical plant has been eliminated, but there is still a continuing threat to public safety.

Some 50,000 residents in Garden Grove, a city of roughly 170,000 about 40 miles (60 kilometers) south of downtown Los Angeles, have been evacuated and are waiting for a resolution.