In late November, authorities found a shiny metal monolith in the middle of the Utah desert. Since then, similar monoliths have appeared in Romania, Spain, Netherlands, California, and even the Isle of Wight. The seemingly random appearance of the monoliths has worried conspiracy theorists, who believe it could be a warning from aliens.
Prominent alien hunter Scott C Waring made the claims on his blog UFO Sightings Daily.
He said extraterrestrials may have begun probing the Earth to see if it is suitable for a future invasion.
Mr Waring wrote: “What does this all mean? Remember when I said one was not an alien invasion?
“Well, three in three different parts of the world is a good start to probing the environment to a future invasion point. This is getting a bit creepy.”
While the monoliths in Europe remain a mystery, an art collective group has claimed responsibility for those found in the US.
The group, known as The Most Famous Artist, is now selling replicas of the monoliths on its website.
However, those wanting a piece of the famous art installation will have to pay the costly sum of $45,000 (£33,820).
The group’s founder, Matty Mo, said the monoliths in the US were planted to cheer everyone up after a miserable year.
The Most Famous Artist collective has also claimed responsibility for several high profile stunts in the past.
On New Years Day, 2017, residents of Los Angeles were shocked when they woke up to find the historic Hollywood sign had been changed to read ‘Hollyweed’.
Artist and videographer Zachary Cole Fernandez, who is seemingly part of the Most Famous Artist collective, said he did it simply for fun.
Mr Fernandez told Buzzfeed News at the time: “It was something to smile and laugh out loud about.
“Just lift their spirits and let them live because 2016 was a crazy year, dude.”