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The ¡°cages¡± as they are called were installed in 2005 by Jeju Fisheries Research Institute and a local fish farm association. The cages are so high-tech that it took the U.S. National Oceanic and Atmospheric Administration 10 years and US$100 million to develop them.
According to Yang Jun-bong, a representative of the fish farm association, although the underwater cages seem to be simple structures, they are actually very complex. In order to keep the pens at a constant depth, like submarines they have to achieve neutral buoyancy -- in which the weight of the object under water is equal to the upward buoyant force of the water.
The pens also had to be specially constructed to withstand high tides and bad weather, and the nets are made of a special fiber designed to last 10 years. They've been nature-tested already: when Typhoon Nabi hit in 2005 and Ewinear in 2006, five buoys above water were lost but little damage was done.
Source: english.chosun.com