A [[concrete]] wave breaker, shaped like a huge caltrop or jack. They are placed in large numbers at the edges of [[harbor|harbors]] or [[beach|beaches]], acting as a reef.

h4.See also
* [[ocean]]
* [[shore]]

h4.External links
* "Tetrapod on Wikipedia":https://en.wikipedia.org/wiki/Tetrapod_%28structure%29
テトラポッド