Tetrapods, as viewed at the outer surface of the west jetty in the Santa Cruz small craft harbor. Each tetrapod weighs about 25 tons. They are necessary to secure the entire jetty structure. They have a Jacks-like look about them. Tetrapods were first developed in South Africa. A design engineer watched a group of rural children playing a game similar to Jacks. They used four-pronged goat knuckles for throwing pieces. The engineer was amazed by the stability of the goat knuckles when thrown on a dirt surface. Huge prototypes, cast in concrete, proved to be stable against harsh weather and erosion. Tetradpods are now used world wide for their stability.