Authorities say Mike Herdman, 36, was not carrying any supplies, and he was only wearing shorts and a T-shirt when he disappeared.
Herdman and his friend went hiking Wednesday deep into the back country along Sespe Creek. As they were setting up camp Friday night, his dog Duke took off. Herdman followed barefoot and disappeared in the remote mountains above Fillmore.
It took Herdman's friend two days to hike out and get cellphone service to get help. He was finally able to alert authorities Herdman was missing on Sunday.
Ventura County Sheriff's Department Sgt. Eric Buschow says search-and-rescue crews have been scouring the mountain in the air and on horseback. Crews have been following every lead possible, including Herdman's footprints.
Herdman's wife, family and friends have all gathered at the command post, waiting for good news about the experienced outdoorsman and athlete. Searchers found Herdman's backpack Monday, and say they don't suspect foul play.
And while there's still hope, sheriff's deputies say the terrain is dangerous.
"It's a big area, there's boulders back there in that creek that are bigger than the vans that are here. If somebody falls and goes into one of those plunge pools or something like that, to say it's a needle in a haystack, it's more than that," Buschow said.
Ventura County deputies say the good news is that there is a creek to provide water, and the temperature overnight will dip into the mid-50s, which is warmer than it was over the weekend. They say the conditions are "survivable," a factor that could help Herdman survive another night in the wilderness. The search will resume Wednesday morning.