It is not just what Google "knows" but what it holds. It holds a rough copy of anything on the Internet! That is what the "cached" link does. So often people remove a page, only to still have it be visible on Google servers. The scale they have is just hard to fathom to hold all of this information and serve it up so reliably and fast.
Founder, Madrona Digital Audio, Video, Home Automation
Contributing Editor, Widescreen Review Magazine