5.2 magnitude earthquake shakes Southern California.

A earthquake rattled Southern California late Monday morning, startling residents from San Diego to Los Angeles

(CNN)- A 5.2 magnitude earthquake rattled Southern California late Monday morning, startling residents from San Diego to Los Angeles and even prompting  reactions from animals at the San Diego Zoo Safari Park. The quake struck just after 10 a.m. near Julian, California, at a depth of about eight miles, according to experts at Caltech. Reports from the U.S. Geological Survey estimate more than 25 million people experienced “weak to light shaking” across the region.

“The whole sky was rumbling,” said one resident, while another added, “There was just a huge earthquake in San Diego. Must have been like a five point something.” That estimate wasn’t far off.

Jayea Watkyns, a student who had been napping in her car, was awakened by the tremor and an alert on her phone. “I just thought it was like a big truck or something,” she said. Sandra Ruiz, who was in her office when the quake hit, said she received a warning seconds before feeling the shake. “It was a pretty good shake,” she noted. Tremors were felt across a wide swath of the state, from Los Angeles to San Diego. Even animals reacted. At the San Diego Zoo Safari Park, African elephants formed what experts call an “alert circle,” instinctively surrounding a calf to protect it during the quake.