Some mysterious underwater circles spotted in the Baltic Sea have been explained, and poison is to blame. But let's back up and start from the beginning.
In 2008, a tourist to the island of Mon in Denmark noticed strange patterns in the shallow waters beneath the island's striking white chalk cliffs. As you can see in the image above, the patterns looked like circles, and they were rather large. Some of them were as much as 50 feet in diameter.
Speculations were far ranging: Were the circles remnants of bomb craters from World War II? Were aliens involved?
Biologists took a closer look and revealed that the strange shapes were created by eelgrass that appeared to be growing in rings. Thick, lush eel grass grows around the perimeter of the circle, but in the center there is either struggling eel grass, or none at all.
But eelgrass usually grows in a meadow formation, so why in this case was it growing in ring shapes?
According to biologists Marianne Holmer from University of Southern Denmark and Jens Borum from the University of Copenhagen, the rings are created because of a buildup of sulfide in the mud around the grass. The sulfide, which is toxic to eelgrass, kills the grass in the center of the rings, but not the grass along the perimeter.
Most of the mud in this area of the Baltic Sea is quickly washed away, but the strands of eelgrass trap the mud in their midst.
"The mud seemed to exist only inside the circle, so only here the plants are attacked by poison," said Holmer and Borum in a statement (in Danish).
Eelgrass grows by stolons, which spread radially in all directions. So what happens is the grass traps the mud, the mud kills the old, weaker grass in the center of the circle, but leaves the newer grass at the edge of the circle to thrive.
The end result? A no-longer so mysterious circular shape in the seabed.
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