Can you imagine being on a deep-sea dive, and coming across something that looks for all the world like a crashed underwater UFO?
That's what happened to some divers from the Ocean X team back in June 2011, exploring part of the northern Baltic Sea.
Swedish explorers Peter Lindberg and Denis Asberg saw radar pings that led them to a remarkable underwater mystery, in the form of a massive 70-foot-long disc of metal or rock.
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The thing looked almost impossible to explain, and called to mind the famous Millennium Falcon spaceship from Star Wars.
Unlike Han Solo's ship, though, it was buried deep under the ocean rather than flying through space. It's since become known as the Baltic Sea Anomaly.
At the time, Asberg reportedly told TV4 in Sweden: "We were really surprised and puzzled. We were thinking what is that we have found here. This is not a wreck. At that time we thought for sure there was a natural explanation.
"We wanted to look into it some more and decided to keep this totally quiet. We contacted geologists, marine biologists and when they said they had never seen anything like this, of course more thoughts were triggered. It could be something really awesome that we’ve found."
So, to give credit where it's due, the people who discovered the Baltic Sea Anomaly weren't for a moment tempted to label it as a bit of alien technology.
Still, once it became public that didn't stop people from jumping aboard and coming up with some pretty fun ideas.
One theory suggested that it was a marker of the lost underwater city of Atlantis, featuring what look like 'stairs' supposedly leading down to the city. Others kept things simpler by insisting it was indeed a crashed and lost UFO.
Adding to the Anomaly's reputation is the fact that it seems to interfere with radio and electric equipment - Asberg said that radios all cut out when they got closer than 200 meters.
Every sample that has been taken from the Anomaly over the years, though, seems to point to the fact that it's made of a combination of metal and rock, and that it's still most likely explained by gradual geological processes.
The circular shape of the the Anomaly might seem absurdly unlikely to have natural causes, but various researchers have insisted that this is still the overwhelmingly likely explanation.
According to NBC, Swedish tabloids quoted Stockholm University geologist Volker Brüchert as saying: "My hypothesis is that this object, this structure was formed during the Ice Age many thousands of years ago."
That's been backed up by further research at Stockholm University, which suggests that the Anomaly was left behind by the movement of glaciers aeons ago.
Still, there's enough unknown here to clearly fascinate plenty of observers, making it one of the weirdest underwater sights on Earth.