Bottomless Pit - Monticello Dam Drain Hole (Lake Berryessa, Napa County, California, USA)

This is not a natural phenomenon, but still extraordinary. 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, Napa County, California. It is used very rarely, but it is quite the sight to see it being put to use.