A small pufferfish builds giant geometric circles in seafloor sand.

A two-meter circular pattern on the seafloor sounds like geology, a diver prank, or something made by a much larger animal. Male pufferfish are the builders. Scientific Reports documented males constructing large geometric sand structures as part of reproduction, with radial ridges and fine sand collected near the center. Later work modeled how these structures can influence flow around the nest. The fish is only about hand-sized, yet the finished structure can be many times wider than its body. It is underwater architecture built fin stroke by fin stroke.

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