Large, four-legged concrete structures used to fortify breakwaters. They are designed to dissipate the force of incoming waves and are used along an estimated 50% of the Japan coastline.
Glossary: Tetrapod
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Tetrapod (テトラポット)
Referenced from: Mirage: Saiai no Anata e
Bibilo: テトラポッド大好き!, Tetrapod (structure) (english)