What a fantastic answer to the mystery of these underwater 'crop circles' found off the coast of Japan. The whole story is over at Spoon and Tamago​.

"While diving in the semi-tropical region of Amami Oshima, roughly 80 ft below sea level, Ookata spotted something he had never seen. And as it turned out, no one else had seen it before either."

It turnes out these mysterious and organic shapes are the work of a small little pufferfish and is probably a mating ritual. ​And in fact it seems it is a sort of nest building, the mounds and ridges are meant to protect eggs from currents.