"It was a boom that was just -- I had never heard anything like that before, it was really weird," said Sherril Stephens.
The explosion was so big it shook the town of Esparto, knocking Stephens off her couch and breaking her front window.
"Course all the neighbors ran out into the street. We didn't know what was going on," said Paul Williams, who heard the explosion.
Some people said they thought it was a plane crash, others, a house explosion.
The smoke and dust rose from the site about a mile from town.
It turns out the Discovery channel's Mythbusters were taping an episode there.
They were trying to literally "knock the socks off" a mannequin by igniting 500 pounds of ammonium nitrate. But the explosion was a lot bigger than they expected.
