A mysterious metal monolith discovered last week in the desert in the US state of Utah has disappeared, officials say.
Utah's Bureau of Land Management said it had seen credible reports the object had been removed "by an unknown party".
Social media images apparently from the site show a pile of rocks and a small piece of metal left behind.
The discovery of the 12ft (3.7m) high object sparked a national guessing game as to how it got there, and saw dozens of people pay a visit in recent days.
The strange metal object was first spotted on 18 November by a helicopter crew counting big horn sheep from the air, in a remote south-eastern area of Utah.
News of its discovery and speculation over its origins quickly went viral on social media, with many observers presuming it was an art installation left by a sculptor.
As yet no-one has claimed responsibility for installing the structure.
Ryan Bacher, a Utah helicopter pilot who visited the site on Friday afternoon, told local TV station KSL that when his friend returned the next day it had gone.
"Twenty-four hours later, my close friend, who is also a helicopter pilot, flew his family down to see as well and found it taken down," he said, adding he would like to know who did it.
The state's Bureau of Land Management (BLM) said in a statement it had not removed "the structure which is considered private property".
"We do not investigate crimes involving private property which are handled by the local sheriff's office," it added.
Several people posted images on social media said to be from the site, posing with what remained of the monolith.
The "monolith" obelisk met a swift fate before we arrived. Only the triangular top remained as well as parts of the below-ground base. If I ventured a guess, it was probably dismantled by someone determined to prevent careless curiosity that tends to ruin alluring sites. pic.twitter.com/kXP5KypmlS— Jack Waters 🌹🇸🇪 (@h2oetry) November 29, 2020
Last week, the Utah Department of Public Safety Aero Bureau released images of the mysterious metal object, but did not disclose its exact location fearing explorers may try to seek it out and "become stranded".
However, this did not stop dozens - perhaps even hundreds - of curious sight-seers from flocking to the site in recent days after internet sleuths were able to track down the monolith's GPS co-ordinates.
But the mystery of its origins remain.
The pilot of the helicopter who made the discovery, Bret Hutchings, speculates the monolith may have been installed by "some new wave artist" or a fan of 2001: A Space Odyssey, the 1968 film directed by Stanley Kubrick.
Imposing black monoliths created by an unseen alien species appear in the movie, based on the writings of novelist Arthur C Clarke.
Officials said it was likely a work of art, but its installation on public land was illegal, "no matter what planet you are from".