Somewhere between 35 and 90 percent of the web has at least one archived copy. That's a pretty big range.
Here's the challenge: new Internet is being made all the time. Oftentimes, these new pages are added to existing networks on Tumblr or Facebook or Twitter or Livejournal. But other times, someone fires up a web server that's off the standard map, and it the web's crawlers, try as they might, may not find that page for a while, if ever.
That means some percentage of the web is not being archived by anyone (or anything, really), not even the Internet Archive's invaluable Wayback machine.
And certainly, few sites are being archived with any kind of regularity, even those (like TheAtlantic.com) that are changing constantly. So, how much of the web is humanity missing?