Did you ever wonder why Gmail is free? How is Google able to cope with googols (yup, it's a word) of mail a day? Well here's the answer! Now you know where Google's been hiding your emails (and why they offer 2.8Gb and counting for free).
Just kidding! It's really how Google sends your mail. How else can they do it?
Got you there! On a more serious note, clips were submitted by users to Google to stitch together the final piece you see up there. See the official Google blog or this special link for more details.