Photographer Yoji Ookata was on one of his frequent dives in the Pacific Ocean when he noticed something unusual this year. Imprinted on the ocean floor, 80 feet below sea level, was a circular geometric pattern, about 6.5 feet in diameter, that had been carved into the sand.

Filled with ridges of varying heights, the pattern looked strikingly similar to a crop circle.

In his 50 years as a professional deep-sea photographer, Ookata had never seen anything like it. He photographed his discovery, and dubbed it the “mystery circle.” The next time he returned to the spot, located off the coast of the island of Amami Oshima in southern Japan, he came armed with a television crew.

And, this time, Ookata and his team caught the artist in the act.

“Using underwater cameras, the team discovered the artist is a small puffer fish only a few inches in length that swims tirelessly through the day and night to create these vast organic sculptures using the gesture of a single fin,” the blog ThisIsColossal reported.

The puffer fish is a small but poisonous type of fish, prized as a delicacy in Japan.

Female fish are reportedly so attracted to the multilayered ridges sculpted in the sand, that they will seek out the male fish upon discovering his so-called nest. After mating, the couple will then deposit the newly fertilized eggs in the sand at the center of the circle.

Besides being beautiful, the underwater crop circle also serves a highly practical purpose: The carefully carved ridges act as buffers against ocean currents, protecting fertilized eggs from being either moved or swept away.

And researchers have also observed that the more ridges built into the structure, the higher the chance the fish will mate. Additionally, the small rocks and seashells, so carefully dragged around the periphery of the circle, can serve as sources of nourishment for the small fish.