Underwater photographer Yoji Ookata has spent 50 years exploring the ocean depths, but the sight of massive underwater patterns resembling the crop circles celebrated by UFO enthusiasts still surprised him.

The "mystery circle" as he called it was more than six feet in diameter and contained intricate patterns of ridges and radiating out from the center. What on earth could have created these amazing structures located 80 feet below the surface of the ocean? Ookata returned to the depths with a TV crew to find out.

As revealed last week in a Japanese television special entitled "The Discovery of the Century: Deep Sea Mystery Circle," the patterns were not caused by aliens or underwater currents but by a tiny puffer fish.

Puffer fish are valued in Japan as a delicacy known as sashimi chiri, which can cause mild intoxication or, in rare cases, death due to an incredibly powerful neurotoxin found in the fish's ovaries and liver. But until now, no one also knew that they were artists.

Ookata and his video crew observed a tiny male fish spending days to make the circular ridges on the ocean floor using only a flapping fin. It involved more than just moving sand around: the fish actually carried shells into the pattern, broke them, and scattered the pieces along the inner ridges of the pattern, according to an account of the television special on the website Spoon & Tamago, which is dedicated to Japanese art and design.

While beautiful, these "mystery circles" also served a purpose: they attracted females who mated with the male and laid their eggs in the center of the circle. The scientists on the mission theorize that the eggs are actually protected by the ridges and patterns, which neutralize currents and make them less susceptible to predators.

The team observed several puffer fish creating these structures, and discovered one more key element to the mystery: the males that created more intricate circles attracted the most females.

As for the seashells, they, too, may be more than decoration. It's possible they provide nutrients to the young puffer fish as the eggs hatch.

Their work is explained in more detail in this study published in the journal Scientific Reports.