![](/nationalgeographic/pioneer_april_2021/nature_s_mysteries_3/1121909/sand_line.960_0_1.png)
Yoji Okatoisanunderwaterphotographer.Oneday,hedoveintotheseaoffsouthern Japan.
Hesawsomethingoddontheseafloor.Itwasacircleinthesand.Thecenterlookedsmooth.Apatternofridgeswaspileduparoundtheedgeofthecircle.Hewonderedwhatmadethis circle.
Searching
foraCause
Peoplecameupwithideasabouttheseseacircles.Somesaidtheyweremadebyoceancurrents.Somethoughtthataliensfromspacemadethecircles.ThealienshadlandedtheirUFOsonthe seafloor!
Okatodidn’tthinkalienswerebehindthecircle.Hewantedtosolvethemystery.Heteamedupwithsomescientists.Theteamreturnedtotheplacewherehehadseenthecircle.Nothinghad changed.
![](/nationalgeographic/pioneer_april_2021/nature_s_mysteries_3/1121916/Minden_Yoji_Okata_copy.480_0_1.jpg)
Yoji Okato
![](/nationalgeographic/pioneer_april_2021/nature_s_mysteries_3/1121917/NGX0421_EE_14B.640_0_1.jpg)
sand circle
![](/nationalgeographic/pioneer_april_2021/nature_s_mysteries_3/1121918/asia.256_0_1.png)
Asia
Japan
![](/nationalgeographic/pioneer_april_2021/nature_s_mysteries_3/1121922/.GettyImages_1073085534.mp4.512_0_1.jpg)
Apufferfishflapsitsfinstomove sand.
Theteamwenttothesameplacemanytimes.Theywatched.Theydidn’tgetanyclues.Themysterywasstilla mystery.
MysterySolved
Oneday,theteamsawwhowasmakingthecircle.Itwasasmallfish.It’scalledapufferfish.The fishmadetinywavesbyswimmingbackandforth.Thewavesmadeacircle.Heusedhisfinstomakeridges.Whenhewasdone,afemalefishswamby.Shelikedthecirclesomuch,shelaidhereggsthere.Thecirclewasanest!Mystery solved!
![](/nationalgeographic/pioneer_april_2021/nature_s_mysteries_3/1121920/.GettyImages_1096054650.mp4.512_0_1.jpg)
Apufferfishswimsbackandforthasitmakesasand circle.