Mystery Seagrass Circles In Croatia Puzzle Experts

You've heard about crop circles, but what about seagrass circles? Aerial photos of the coastline of several islands in Croatia show regular circles of sand amidst a sea of Posidonia oceanica, a seagrass endemic to the Mediterranean.

Biologist Mosor Prvan from the Sunce Association, a nonprofit environmental organization that first noticed the circles, doesn't have an explanation for the phenomenon. The circles are about 164 feet in diameter and are roughly at the same distance from the islands and from one another.