3:45 p.m. Update:
The Madera County Sheriff's Department reports the gas leak has been repaired and Highway 99 has been reopened.
CBS47's Diane Tuazon reported at noon that the highway will be closed for at least several more hours.
The official cause has not been announced but it has been reported that some sort of heavy machinery hit a 12" gas line around 10:00 a.m. Friday morning causing a large leak.
The leak forced a half mile evacuation of the area, including all lanes of the highway.
Eric Stuart with the Madera County Sheriff's Department said, "It's bad, it's a serious leak. As to what caused it, we don't know. The problem is where it's located, which is forcing us to shut down 99 between 12 and 7."
Firefighters are concerned a spark could ignite the gas that could still be heard escaping 2 hours after the accident.
Traffic is backed up for miles on the busy highway and is being rerouted around the area.
Highway 99 is closed in both directions between Avenue 7 and Avenue 12 because of a gas leak.
The California Highway Patrol has not said when the road is expected to reopen but they are advising the public to use an alternate route and avoid the area.
CBS47 has a crew on the seen and will continue to monitor the progress.
For latest road conditions, call 1-800-427 ROAD (7623) or visit www.dot.ca.gov
and click on Current Traffic Conditions.