Sightings of similar structures are taking social media by storm, as images of them started appearing over the last two weeks.
The first monolith was spotted by a helicopter in a remote area of the Utah desert. It has since been removed.
Another recently appeared in Romania.
It's unclear who set up this latest monolith along the California central coast or why the strange columns continue to appear.
"I think it disappeared in Utah and landed right here in Atascadero," said hiker Blake Kuhn.
The sightings are drawing comparisons to the film "2001: A Space Odyssey." But experts have said it is unlikely these objects are the work of aliens.
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