And the winner is:

Ok, since no one else has decided to make a guess about the concrete structure in the shots below, I should just tell you.

Sam is technically correct. It's a drain-hole from a man-made lake at the top of a mountain near Atlanta, GA. Water drains into the hole, comes down 600 ft. and drives a hydroelectric generator turbine and exits into a bottom lake. This drain is roughly 30 ft. in diameter and 600 ft. straight down. It was blasted out of solid limetone/sandstone.

The question is, how do they get the water back up to the top!? Well, during the day, power is expensive, so they run the generator and water comes down. At night, power is cheap and in excess from nearby nuclear stations so they take that power and drive the turbine generators in reverse as motors and push water back up to the top lake. By the next morning it's ready to go again. As they run these 343,000 hp motor/generators in reverse, the 16 storey, mostly underground concrete bunker structure shakes from all the energy dissapated from the turbines.

The drain is capped like many are in bathtubs to avoid creating cyclones in the water as water goes into the drain.

Cyclones = excess air in water = destroy turbines

Although Sam was right, Cal, wins the prize as I'm sure that:

1. Cal can eat that much food

2. Cal can clean his plate like that