Debris from a passing alien spacecraft or a sculpture put there by a mischievous artist a decade ago? While ufologists and conspiracy theorists were this week debating what the Utah monolith – a silver pillar that mysteriously appeared in the US desert last month – actually is, an "unknown party" removed the object, and got the world talking about it yet again.
“We have received credible reports that the illegally installed structure, referred to as the 'monolith' has been removed from Bureau of Land Management public lands by an unknown party,” the US Bureau of Land Management agency said in a Facebook statement. “The BLM did not remove the structure, which is considered private property.”
The exact location of the monolith hadn't been disclosed to the public, to prevent visitors from becoming stranded in the remote area, but people found it – and posed with it – anyway.
The New York Times reported an Instagram post by the Utah Department of Public Safety, which declared: "It's gone! Almost as quickly as it appeared, it has now disappeared," adding, one "can only speculate", alongside the alien emoji. The post has since been removed.
But while this mystery seems to have lasted no more than a fortnight, other unsolved conundrums have endured for over 100 years and counting.
Here are five to keep you up at night…
1. The Voynich manuscript
The manuscript is an illustrated, handwritten codex scribed in a writing system and language seemingly unknown to anyone on Earth. Written on vellum (fine parchment made from calfskin), which has been carbon-dated back to the early 15th century, experts believe it is Italian in origin. It is named after Wilfrid Voynich, the Polish book dealer who bought it in 1912.
Cryptographers throughout history have had a crack at the manuscript, which still has 240 of its remaining pages intact, with both American and UK codebreakers from the First and Second World Wars drafted in to make sense of the script ... but it has so far eluded decrypting.
2. The disappearance of the Flannan Isle lightkeepers
What happened to the three lighthouse keepers of Flannan Isle in the Outer Hebrides off the coast of Scotland in 1900?
On December 7, James Ducat, Thomas Marshall and William MacArthur alighted on the island to look after the lighthouse for a two-week shift. However, on December 15, the steamer ship Archtor recorded something amiss. Sailing past in foggy weather, the captain made a note that said lighthouse was without light.
Owing to adverse weather, no relief vessel could reach the island until December 26. When the rescuers arrived, they found the lighthouse locked, the clocks stopped and, strangely, the men's waterproof gear inside … but no lighthouse keepers. No bodies were ever found.
Another code no one has been able to crack is the fourth one on Kryptos, the coded sculpture that stands outside the CIA headquarters in Langley, Virginia. Created by US artist Jim Sanborn and unveiled in 1990, the sculpture has eluded even the brightest minds at the Central Intelligence Agency, who have not been able to break the final code that Sanborn carved into the copper and granite piece. The first three, however, have been solved over the years by computer scientists and members of the CIA and National Security Agency.
Back in 2006, Sanborn revealed that the answers to the first three passages contain clues to the fourth, but as of yet, no one has been able to solve it. Fitting, really, as the name "Kryptos" comes from the ancient Greek word for "hidden".
4. The mystery of the 'USS Cyclops'
On February 16, 1918, the USS Cyclops, a Proteus-class collier built for the US Navy before the First World War, left Barbados with 306 passengers and crew. Bound for Baltimore in the US, she was never seen again. As the ship was travelling during wartime, it was assumed she had been the victim of German U-boats, as she had been carrying 11,000 tonnes of manganese ore, which is used to produce munitions.
However, German authorities at the time, and ever since, have denied they had anything to do with the disappearance of the vessel. The USS Cyclops had travelled through the Bermuda Triangle after leaving Barbados, and remains the single largest loss of life in US naval history, not involving combat.
5. The Dyatlov Pass incident
Many theories have been put forward as to what happened on the slopes of the Ural Mountains on the night of February 1, 1959, which resulted in the bodies of nine experienced Russian hikers being found under inexplicable circumstances. But nothing conclusive has ever emerged, despite numerous investigations.
It has been established that during the evening of February 1, something occurred that led the men to tear their way out of their tents and race from their campsite wearing inadequate clothing in sub-zero temperatures.
Soviet authorities determined that six died from hypothermia, while of the other three, one had a fractured skull and the other two had major chest fractures.
The investigation concluded that a “compelling natural force” had caused the deaths, although there was no evidence of an avalanche. Other theories that have been put forward over the years include animal attack, infrasound-induced panic, katabatic wind, military involvement or human attack.