Off southern Japan, mysterious large circular patterns have appeared on the sandy seabed near Amami Oshima Island since the 1990s. These shapes, about two meters wide, puzzled divers. They seemed too perfect to be natural. After years of mystery, researchers discovered the maker: a small male pufferfish of the genus Torquigener. Measuring only 10 centimeters, the pufferfish builds these circles as nests over about a week. It swims back and forth to dig ridges and grooves like spokes on a wheel. It flattens the center and decorates the ridges with shells and coral fragments. This artistic creation attracts female fish. "Females only visit when the structure is complete," scientists note. The male shows off by stirring fine sand, creating a visual display. Eggs are laid in the calm center area of the nest, which helps protect them from water flow. After spawning, the male guards the eggs for days but does not fix the nest. Currents erase the structure over time, and once the eggs hatch, the male moves on to build a new nest elsewhere. Studies using water flow models find the nest slows currents by over 20%, keeping eggs safe and helping fine sand gather inside. These seabed circles highlight complex animal behavior underwater. According to scientists, the nests show how sexual selection drives precise construction in fish, much like in birds and insects. Each year the sand art vanishes and is reborn, a quiet yet remarkable ocean story.