The mysterious monolith discovered in the Utah desert just over a week ago has randomly disappeared.
The Utah Department of Public Safety discovered the 3-metre-tall object while counting bighorn sheep from a helicopter.
"One of the biologists is the one who spotted it and we just happened to fly directly over the top of it," pilot Bret Hutchings told local news station KSL-TV.
"He was like, 'Whoa, whoa, whoa, turn around, turn around!' And I was like, 'what?' And he's like, 'There's this thing back there - we've got to go look at it!'"
The discovery sparked an internet challenge as people tried to locate the structure, but those looking now will be out of luck.
On Sunday Utah's Bureau of Land Management (BLM) said in a statement they had received reports the monolith had been removed by "an unknown party".
"The structure has received international and national attention and we received reports that a person or group removed it on the evening of Nov. 27."
The structure was considered private property and crimes involving private property are handled by the local sheriff's office, the bureau said.
Reacting to the news in an Instagram post, the Department of Public Safety exclaimed: "IT'S GONE!", The New York Times reported.
"Almost as quickly as it appeared it has now disappeared. I can only speculate (that aliens took it back)."
It was already a mystery how the monolith had been installed in the first place. Past satellite imagery suggests it was installed sometime between August 2015 and October 2016 and it isn't hollow - it appeared to be made up of steel sheets riveted together.
The most likely theory is it may have been the work of New Mexico artist John McCracken who was fond of science fiction.
His son Patrick McCracken told The New York Times his father told him in 2002 "he would like to leave his artwork in remote places to be discovered later".