Yoji Okato pulled on his dive gear. The underwater photographer gathered his waterproof cameras. Then he dove into the sea off southern Japan.

When he reached the seafloor, he saw something odd. It was a giant circle in the sand that was two meters (six feet) wide. The sand in the center looked smooth. A pattern of symmetrical ridges piled up around the edge of the circle.

Okato took pictures of the sand sculpture. When he shared his photos, other divers said they’d seen these mysterious circles, too. Yet, no one had ever seen what made them.

People had theories about these sea circles. Some said they formed when a UFO landed on the seafloor. The spaceship left a mark in the sand, they said. Others thought ocean currents sculpted the sand. The moving water could push the sand into ridges. Yet, the circles seemed too perfectly round to be formed by shifting water currents.

To solve the mystery, Okato teamed up with scientists and filmmakers. Time after time, they returned to the dive site. The team watched and waited to see how the circles formed. Dive after dive, they saw nothing and swam away without any clues.

Finally, the team caught the artist in the act. It wasn’t an alien, and it wasn’t an ocean current. It was a male pufferfish. He swam back and forth. The motion made tiny waves. These waves moved the sand, forming a circle. Then the fish used his fins to dig into the sand. He flipped fine sand into the center of the circle. He dug dips and built ridges around the edge. Finally, he collected bits of shells in his mouth and carefully placed them on the ridges.

Japan