It is called "Glory Drain Hole" Glory holes are used for dam's to drain excess water during dry seasons. This is the largest one of them all in the world, it is located in Lake Berryessa.
It is used very rarely, but it is quite the sight to see it being put to use. Located in Napa County, California, USA this glory hole was constructed between 1953 and 1957. The hole diameter is 72ft (22m) wide whilst the critical drop is 280ft (86m).
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