This is one of those ideas where some person was like “Hehe, this might be silly.” And then struck fucking gold.
These amazing underwater “crop circles” are the result of a species of pufferfish. The males create these large, intricate designs in the seabed with their fins to attract potential mates. The females inspect the finished design and decide if they wish to mate with the male from there. The female will lay her eggs in the center of the circles, and the male will then fertilize and guard the eggs.
Previously, the circles were of unknown origin, and remained a mystery for several years until it was discovered that these little guys were responsible for the beautiful designs.