Two mystery monoliths, similar to those that have appeared in the US, Romania and the UK, have now shown up in Poland.
The first was spotted in the southern city of Kielce at the site of a former quarry turned into a nature reserve, while the second was found on the banks of the Vistula river in Warsaw.
They are just the latest to appear in a puzzling international mystery that has so far reached at least nine countries.
Standing at around 9-ft tall, the mystery object at Kielce was discovered on Wednesday morning by an employee at the site.
Andrzej Płonka from Geopark, which now manages the Kadzielnia reserve, told local media: ‘On Wednesday morning, our employee discovered a strange monolith in the excavation in Kadzielnia.
‘It is made of metal, and has been solidly and professionally made.
‘It is about 2.5-3 meters tall. We don’t know exactly when it appeared, either Tuesday after dark or at dawn.
‘Nobody saw anything. Unfortunately, there is no monitoring in this place.’
The second monolith appeared on the banks of the Vistula river in the capital, Warsaw, on Wednesday afternoon.
Posting on social media, a spokesman for the district said: ‘Each trip to the Vistula is an amazing adventure.
‘Crossing the river by the Świętokrzyski Bridge on the right bank, a mysterious and unusual installation emerged from the shore.
‘You can’t miss it.’
Sergiusz Kieruzel, spokesman for Polish Waters Authority said: ‘This is definitely not our initiative. We will be checking whether there was a permit to set up such a facility.’
Meanwhile, a jogger told local media that she was running on her usual route along the Vistula on Wednesday morning when she came across the monolith.
‘It is identical to the monoliths in the USA and Romania, all steel, three meters tall! Anybody knows what’s going on?’
Another visitor was less interested in the mystery, however, telling AFP news agency: ‘I was expecting some kind of metaphysical experience, but it’s not really all that impressive’.
The Polish monoliths are the latest to be discovered after a similar structure first appeared in the Utah desert last month, sparking comparisons to those seen in the Stanley Kubrick film 2001: A Space Odyssey.
Others have since popped up across the US, as well as in Colombia, Romania and the Netherlands.
Prior to the Polish structures, a monolith appeared on the top of a hill in Glastonbury on Wednesday with the words ‘Not Banksy’ etched onto it.
Walkers discovered the large silver structure on top of Glastonbury Tor – an ancient hill linked to King Arthur and celtic mythology.
It was believed the monolith had been placed there overnight – but was felled by a gust of wind.
The shiny triangular pillar featured a stencil drawing of a rat, similar to the style used by street artist Banksy.
Prior to that, another popped up on the Isle of Wight on Sunday, drawing a crowd of locals looking to take a selfie with the unusual object.
Designer Tom Dunford later admitted that he installed it as a tribute to others around the world.
Structures also appeared this week in a small German town, a forbidding Spanish hillside and a muddy potato field in Belgium.
One of the bleak silver columns showed up in Sulzbach, Germany, overlooking a field near a shopping centre.
Another was by the ruins of an old church in Ayllon, Spain, where local authorities warned people to steer clear of the dangerous slopes – while a third was spotted on Tuesday in the Flemish village of Baasrode.
While the people behind some of the mysterious structures have been revealed, for the most part their origins remain a mystery.
Theories abound, including suggestions that artists, aliens or savvy marketing executives.
That some of the monoliths were quickly removed has only added to speculation about their potential origins.
The first shiny pillar was spotted in southern Utah on November 18 by baffled locals and news of the object quickly went viral around the world.
Some observers pointed out its resemblance to the avant-garde work of John McCracken, a US artist who lived for a time in nearby New Mexico and died in 2011.
But McCracken’s representatives have given ambiguous and at times conflicting responses to this theory, prolonging an international guessing game that intensified further with the monolith’s sudden removal on Friday.
The Most Famous Artist, also known as Matty Mo, is also thought to have been behind some of the monoliths that appeared in the US.