When a team of underwater explorers discovered a mysterious man-made object 91 metres under the Baltic Sea in Europe, they were stumped as to what it could be.
Five years later, it seems that no-one has come any closer to identifying the huge object on the ocean floor.
Dubbed the 'Baltic Sea Anomaly', the structure has been likened to an alien spaceship due to its round shape and futuristic looking lines.
The Swedish team behind Ocean X made the discovery in 2011 and are now fundraising to make a second voyage into the deep.
Professional diver Stefan Hogerborn said in a bizarre twist, electronic equipment would fail when the crew passed over the unidentified object.
“Anything electric out there, and the satellite phone as well, stopped working when we were above the object,” Mr Hogerborn said.
“And then when we got away about 200 metres, it turned on again, and when we got back over the object it didn’t work.”
The huge object measures 61 metres wide and eight metres tall, prompting many to draw their own conclusions as to what it could be.
Some have speculated that it could be a sunken Russian ship. Others have said a battleship gun turret or a Nazi anti-submarine device.
Of course, paranormal enthusiasts whole-heartedly believe it is an alien craft.
Samples of the surrounding area taken by the Ocean X team and given to geologist Steve Weiner found the object was definitely man-made.
He found the materials collected in the samples were "metals which nature could not reproduce itself".
The crew need to raise $360,000 before they can set out on their next expedition to the anomaly.