How to search the invisible web

The invisible web refers to web pages not indexed by search engines such as Google. Various sources say there is more information on the invisible web than the surface web. One way to access unindexed pages is to use Google to find databases related to your search topic. Then you can search the databases for information that Google or other search engines don't index.

Let's say you want to search the Web for information on "snake venom antidote."

(Bystander search strategy doesn't lead to invisible web resources.)
--Google search term: snake venom antidote / 103,000 hits

(Sidekick search strategy leads to invisible web resources, but it's a lot to sift through.)
--Google search term: snake venom antidote database / 30,600 hits

(Hero search strategy leads to invisible web resources and narrows the results. Now you can search further by accessing one of the 67 databases.)
--Google search term: intitle:database snake venom antidote / 67 hits