The estimated size of the open web – what you search when you use Google – is 167 terabytes. And while that's clearly a very large number, the estimated size of what's called the Invisible Web, or Dark Web, is 91,000 terabytes. This deep end of the search ocean contains databases and scholarly libraries such as Library of Congress holdings, not able to be accessed by search crawlers.
This article lists several tools that you can use to explore this vast body of information. Be sure to explore the various advanced filtering options available, because these waters run deep:
To read more about the Invisible Web, check out the collection of articles at the end of this Wikipedia page: