Authorities said both the on and off-ramps for southbound Highway 101 at River Road have been shut down and diverted.
Sonoma County Sheriff's Office Sgt. Spencer Crum says they're unsure exactly what it was right now, but the bomb squad is on scene.
The device was reportedly small, about 18 to 24 inches in length -- two pieces of metal separated by some kind of wire.
@sonomasheriff bomb squad en route to PGE substation off River Road. Suspected device outside fence next to now closed Hwy 101 exit. “It does not belong there,” said deputy of device. #abc7now pic.twitter.com/Xhcx7O147t— Wayne Freedman (@WayneFreedman) October 5, 2018
It was discovered by a worker around 4 p.m. Thursday outside of the fence.
Officials say it's too early to determine who put the device there.
Sgt. Crum says it appears to be some sort of explosive device so they used caution to deal with it.
Officials say there is no danger to the public.
ABC7 News reached out to PG&E but have not yet heard back.
Stay with ABC7 News for more on this developing story.