"The Deep Web covers in the vicinity of 1 trillion plus pages of information located through the world wide web in various files and formats that current Internet search engines cannot find or have difficulty accessing. The current search engines locate and provide access to hundreds of billions of pages at this time." (Marcus P. Zillman, "Deep Web Research 2011")
A simple definition of the "deep web" is what you cannot find with search engines (i.e. Google, Bing, etc). Most often these are databases (especially subscription databases that libraries pay a fee to offer access for the campus community). The "Why" it's hidden information: "when you search in a library catalog, article database, statistical database, etc., the results are generated "on the fly" in answer to your search. Because the crawler programs cannot type or think, they cannot enter passwords on a login screen or keywords." (UC Berkeley - Finding Information on the Internet: A Tutorial)
Use the library's Research Guides to find specialized databases for your subject area. Take advantage of all the sources that are available to you as a Mount Holyoke College student; it's worth it!
Identify the source (or organization) that you need and go directly to that website. This is especially helpful for government information. if you need environmental data, search the EPA (Environment and Protection Agency) website directly. Try using directories of government agencies, such as the A-Z Index of U.S. Government Agencies or use the U.S. Government Information Research Guide.
Locate databases by searching a subject term and the word "database" (i.e. "toxic chemicals database" or "earthquake database").