In certain tracts of ocean, divers know to look for “underwater crop circles,” ornate symmetrical patterns temporarily carved into the sandy sea floor. But since 1995, when they were first discovered in Japan, no one could explain these phenomena. Now, the mystery has been solved: pufferfish are the culprits. The team of researchers behind the finding declared that the “huge geometrical structures” play a role in this species’ mating rituals.

Males, LiveScience explains, create the structures to attract females.

Males laboriously flap their fins as they swim along the seafloor, resulting in disrupted sediment and amazing circular patterns. Although the fish are only about 12 centimeters (5 inches) long, the formations they make measure about 2 meters (7 feet) in diameter. It takes about seven to nine days for the pufferfish to construct the circles.

Although some other fishes construct mating mounds, the pufferfish’s creation is unique for a number of reasons:

First, they involve radially aligned ridges and valleys outside the nest site. Second, the male decorates these ridges with fragments of shells. Third, the male gathers fine sediments to give the resulting formation a distinctive look and coloring, Kawase said.

Females base their decision about whether or not to mate with a male upon his construction skills, although the researchers still don’t understand what it is, exactly, that females are looking for in their ideal circular pattern, LiveScience says. If things go well, however, the female will lay her eggs in the center of the circle, and then, like most fishes, the males will fertilize those eggs externally.

There is a chance that it’s only the fine sand the females are after, not the formations’ intricate patterns or symmetry. “The beautiful lines and structure could serve only to channel those particles to the center, and have no aesthetic purpose,” one of the researchers told LiveScience.

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