The recent Godzilla reboot was probably one of the loudest movies I've seen recently.
One of the most iconic sounds is Godzilla's roar, which is probably one of the most recognizable sounds in cinema.
The sound designers for the reboot wanted to retain the original sound, but they added a few elements from the most interesting sources.
Unfortunately this Wired article doesn't have the isolated audio samples, but it's a mix of a crate being dragged across the floor, dry ice on a metal vent that vibrated, and crunching rocks. They would then add effects to each sound and combine them into the ear blistering scream heard in theatres.
To test out this roar, the designers took outside and businesses from 2 miles away were calling asking what that was.
It's pretty incredible what sound designers will do to get the best results.
Film Dweeb Link: http://goo.gl/aPpU8Q