Carlos Rubio was with his son at Lake Ralphine in Santa Rosa last weekend when he got video of what appears to be a large turtle about the size of a spare tire.
Some believe It could be a species of snapping turtle, a rare find in the state. They can also be dangerous if you get too close.
"At first I thought it was a boulder moving in the water. I realized it was a pretty big snapping turtle," said Rubio.
"They get really large. They're prehistoric," said reptile expert Wendy Rozonewski.
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Rozonewsk said it could be a large snapping turtle or even an alligator snapping turtle.
"They'll eat anything -- I mean anything. If you put your foot in front of it, it will bite you," Rozonewski said.
Whatever it is, it doesn't belong here. Snapping turtles are non-native to California and illegal to possess without permits.
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The California Department of Fish and Wildlife says it plans to investigate. Officials say there's been just 25 to 30 reported sightings of snapping turtles in California and about one in Sonoma County.
Recently, the world was introduced to an alligator snapping turtle nicknamed "Chonkosaurus" that was spotted on the Chicago River.
Giant snapping turtle dubbed Chonkasaurus spotted on Chicago River
The turtle in the lake is likely smaller, but it's creating a big stir about what lies beneath.
"There are things in there we probably don't know about that are living here," said Cary Port.