The big rock smashed through Monica Dalto's sun roof and punctured her skull, causing her arm to go numb. She said she was also could not see anything for about a minute.
Dalto is recovering, but she and her husband Brian Dalto, who was driving the car at the time, are convinced it wasn't accident.
"It was perfect timing that we were going under the underpass. It had to happen right there, so I'm 99 percent sure it was somebody throwing something off the Wendy Drive overpass," said the victim's husband said.
Investigators are trying to track down whoever might have done this.
Anyone with information is urged to contact the CHP's Moorpark office at (805) 553-0800 or 1-800-TELL-CHP.