Nevada Powerhouse

On the Nevada side at Hoover Dam, USA. There's an analogous structure on the Arizona side as well. All the large round structures are electrical generators; if the light on top is lit, the generator is operating. The US flag is hanging off one of two large moveable cranes that can travel along rails on either side of the powerhouse. The cranes, working together, are capable of lifting out the whole generator rotor for repair or maintenance.
This is on the dam tour.