A series of mysterious circles have been pictured in the sea off the coast of Croatia.
The 28 seagrass circles were discovered off Dugi Otok Island in the Adriatic Sea, Like Croatia reports.
They are each 50 metres in length and 300 metres away from each other.
Biologist Mosor Prvan from non-profit environmental organisation Sunce Association told Mashable they are circles of sand among Posidonia oceanica, a seagrass endemic to the Mediterranean: "We've first seen the circles in aerial photos in 2013 while working on a habitat mapping project at the islands of Unije, Susak and Srakane.
"At first we thought it was some sort of photo manipulation, so we dived at one of the locations, and sure enough, the circles were there. They had a perfect edge, as if someone pulled the Posidonia out with a corkscrew."
It's not the first time mysterious objects have been found in the sea. In 2012, a Swedish diving team investigating a mushroom-shaped object at the bottom of the Baltic Sea had come no closer to understanding what it was or where it came from.
The object, which some say resembled the Millennium Falcon, was raised about 10-23 feet above the seabed. The diving team found a 985-foot flattened 'runway' leading up to the object, suggesting that it skidded along the seabed before it stopped.
The object was first found in 2011, but divers were not able to revisit the site until 2012 due to a lack of funding.
Diver Peter Lindberg said: "First we thought this was only stone, but this is something else."
He explained that the object had "rounded sides and rugged edges". It had a 60m diameter and, at the centre, had "an egg shaped hole leading into it from the top."
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