WTF?! In what sounds like the beginning of an alien invasion movie, a third mysterious monolith has been found, with no clue as to how it got there. The object, located atop a mountain in California, resembles the now removed one in Utah and a similar structure in Romania.

The strange story began when the Utah Department of Public Safety's (DPS) Aero Bureau, working with the Utah Division of Wildlife Resources, noticed "a metal monolith installed in the ground in a remote area of red rock" while counting bighorn sheep.

Internet detectives used Google Maps to show it appeared sometime between August 2015 and October 2016, and while the location wasn't revealed, it was eventually tracked down by adventurers.

Following the Utah structure's removal, a similar-looking structure appeared in Romania. Now, a third monolith has appeared on the top of Pine Mountain in the town of Atascadero, on the central California coast.

"The three-sided obelisk appeared to be made of stainless steel, 10-feet tall and 18 inches wide. The object was welded together at each corner, with rivets attaching the side panels to a likely steel frame inside," the Atascadero News reported.

The paper notes that unlike the Utah original, the California version was not firmly set into the ground and could likely be pushed over.

The removal of the Utah monolith had been a mystery, but since then an adventure tour guide, Sylvan Christensen, and BASE jumper Andy Lewis, aka 'Sketchy Andy,' have claimed responsibility. They told the Daily Mail that the pair and two accomplices decided to remove the structure as it attracted tourists who were "destroying" the land around it. The move has seen the men branded as "the Karens of the hiking world" by many on social media.