As he descended to the seafloor in one of his favorite dive spots, his eye was drawn to something off to the left. With a couple strong kicks, he closed the distance, causing the feature to fill his field of view. The gears in his brain turned, processing the visual data pouring in, but the search returned no explanation. He was looking at an intricate pattern carved into the seafloor sediment, perfectly circular. Too geometric to be created by a flopping or scratching creature. Too deep and remote to have been made by some drunken prankster. What did that leave? A chill ran down his spine…

Sounds like the opening sequence to the latest Hollywood sci-fi offering about secret, ancient civilizations or alien visitors (or possibly both). Well, it’s not, but feel free to grab some popcorn.

The circle is real, and it was laid out by an ambitious puffer fish off an island south of mainland Japan. After photographer Yoji Ookata stumbled upon it, he dragged an NHK camera crew out to the site, leading to a program with the over-the-top title of, as translated, “The Discovery of a Century: Deep Sea Mystery Circle” (photos available, but no video). They found that a male puffer fish created and decorated the structure to attract an egg-laying female. That behavior is strikingly similar to bowerbirds, as University of Chicago evolutionary biologist Jerry Coyne noted on his site.

A more detailed English-language account can be found on a Japanese design blog called Spoon & Tamago.