![Bottomless Pit in USA Bottomless Pit in USA](http://i.imgur.com/061a1CG.jpg)
Bottomless Pit - Monticello Dam Drain Hole in in 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.